Why did Leonardo DiCaprio won Oscar after so many years?

Yes he did. There is just one word to it: Perseverance. This is the lesson we can all learn from Leo: When you lose big, and we all do at some point, at the Oscars or not pretend you didn't, and then continue working your ass off.

2016 was his best chance of winning till date, even though I don't feel The Revenant Movie was his best performance and let's be honest: the 2016 Oscars field is hardly vintage. Matthew Mcconaughey's performance in Dallas Buyers club was his career best performance and was definitely better than Leo's performance in Wolf of Wall Street. He lost to people like Jamie Fox, Forest Whitaker and Tommy Lee Jones. While its debatable if any of these actors have careers comparable to Leo's, they gave the best performance of their lives in their corresponding years.

Since Sci-fi & comedy is very dangerous combination to have for the Oscars I think Matt Damon was in the same situation Leo was in 2013. Micheal Fassbender peaked early forSteve Jobs, and no one even paid attention to Tom Hardy’s phenomenal work in Legend. Bryan Cranston as blacklisted screenwriter Dalton Trumbo who was more or less a lone wolf performer in the movie and Eddie Redmayne as transgender pioneer Lili Elbe appeared more serious competition but DiCaprio remained the man to beat, Eddie Redmayne was the only guy who practically stood in between Leo and an Oscar as far as I was concerned. But he’s got two strikes against him, First he won the Academy Award last year for The Theory of Everything. Yes it's not a commendable reason but only Tom Hanks has won the lead actor consecutively in the last 70 years. So the odds were pretty much against him. Second, the best thing about The Danish Girl is Alicia Viakander.
Credentials: Make a mental list of four films that you think DiCaprio could have
earned an Oscar nomination for. For example, would be Gangs of New YorkCatch Me If You CanThe Departed and Inception. But go ahead and compare those to the films he was actually nominated for: What’s Eating Gilbert GrapeThe AviatorBlood Diamond, and The Wolf of Wall Street.
Now let’s check out his credentials. We want to know, essentially, how many good movies DiCaprio has made and how they stack up against the pre-Oscar resumes of those who have been honored with a top acting prize.

Through 2015, DiCaprio has made a movie rated 75 percent or higher about once every 20 months, the same rate as Robert De Niro when he won the 1980 best actor Oscar for Raging Bull. But DiCaprio rates only a hair better than Matthew McConaughey did when McConaughey won the 2013 best actor award. After his win in 2016, he is tied for the nearly 33rd-most-successful pre-Oscar career.
Leo's Terrific in The Revenant: As Hugh Glass, a wilderness guide left for dead by fur trappers, he communicates suffering and tenacity in every small gesture. He holds the film, with its epic two-hour and 36-minute running time, on the strength of his eyes – which, for a good swath of the film, are all he can move. He's nothing short of mesmerizing, even in the face of The Revenant's brutal violence and Emmanuel Lubezki's breathtaking cinematography. Arguably, his is indeed the very best performance of the year.
Physical hardship: The Academy likes a winner who really works for it. He clearly overcomes the other nominees in this category. He is mauled by a bear, sleeps in a dead horse, eats raw bison liver, swims in freezing river rapids, scales up rocky cliffs  turns into a human icicle. After all it's said that fortune favors the brave.
Strong Film: A great performance isn't always enough; you need a great film to
showcase it. I think Steve Jobs and Trumbo lacked this relatively. The movie garnered very strong review and is the kind of technically superb, dramatically gripping epic beloved of the Academy. DiCaprio shines amid other excellent turns from Will Poulter and fellow nominee Tom Hardy.
Road to the Oscars: When it comes down to matching up with eventual Academy Award winners, the SAG Awards are probably the single best barometer of the year and that's all because of who's doing the voting. While the Golden Globes and other critics' prizes are important for getting a general sense of popular sentiment, there's one huge flaw with using them as predictors for the Oscars: there are no journalists in the Academy, which means there is zero overlap between those voting bodies. So as seen from the study by FiveThirtyEight, Leo racked up major awards prior to the final showdown at Dolby Theater and the SAG award gave him a very good boost and made him the eventual front runner.
Question whether Leo was due for an Oscar: Yes, Leo was due for an Oscar so was 22 other living actors. But claiming that The Academy finally decided in favor of him probably due to that reason is not a correct assessment of the facts.
In Oscar history there are 51 performers, DiCaprio included, who have been nominated three times or more for an Academy Award in the acting categories yet have never won. Of those 51 performers, 28 are still alive. Thirteen of the 28 performers have matched or beaten DiCaprio’s 14 films rated 75 percent or higher. And 23 of them have bested DiCaprio in the 90 percent category, having made three or more such highly-regarded films. Let’s not be reckless, though: Leo is a young man. Some of these performers have been at it for decades. Let’s normalize things to a rate of films per year. 22 living actorsat minimum can claim they deserve an Oscar more than DiCaprio, by one methodology or another. 22 performers who were nominated three times or more have acted in films rated 90 percent or higher more often than Leo. Eight of those performers have also acted in films rated 75 percent or higher more often than Leo.

Note: Matt Damon never won an Oscar for acting, the one time he won was for Screenwriting. Similarly Brad Pitt won once for Best Movie.
You could make a lot of arguments about actors outside of this set who probably deserve Oscars based on their past work. But here’s what’s absolutely crazy about this year and the narrative that’s built up around the best actor race. Let’s say DiCaprio deserves an Oscar. You know who deserves one more? Matt Damon, a guy who’ was also nominated for best actor this year! So claiming The Academy finally decided in favor of leo solely because he was due for an Oscar is absurd.

So, in short, Leo was not only well poised to win the Oscar this year, he actually deserved it. After all, man does not live by raw bison liver alone.