Screenwriter Edgar Wright has a personal affinity for Scott Lang
From an interview discussing the movie when it was in development:
…I’d met with Artisan and at the time, they had some of Marvel’s lesser-known titles, and they asked if I was a Marvel comics fan, and I said that I always was a Marvel Comics kid, and they said, “Are you interested in any of these titles?” The one that jumped out was “Ant-Man” because I had the John Byrne “Marvel Premiere” from 1979 that David Micheline had done with Scott Lang that was kind of an origin story. I always loved the artwork, so when I saw that, it just immediately set bells going off kind of thinking going “Huh, that could be interesting. ”
Wright also wanted to do an atypical superhero movie in a different genre
Ant-Man was basically doing a superhero film in invert commas, and it takes place in another genre, almost more in the crime-action genre, that just happens to involve an amazing suit with this piece of hardware. The thing I like about Ant-Man is that it’s not like a secret power, there’s no supernatural element or it’s not a genetic thing. There’s no gamma rays. It’s just like the suit and the gas, so in that sense, it really appealed to me in terms that we could do something high-concept, really visual, cross-genre, sort of an action and special effects bonanza, but funny as well.
I imagine that an Ant-Man movie focusing on Hank Pym would be more of a science-fiction movie, or at least more akin to something like Iron Man. Having someone who is not an inventor, but rather a criminal, means that there is a possibility to add cross-genre crime elements. We get scenes involving breaking into places, rather than discussing "Pym particles" (analogous to gamma rays, which Wright said he didn't care for).
As a side note, I see no evidence that Wright felt Hank Pym was controversial or otherwise unfit for being a hero. In fact, he brings up Hank Pym in the interview (whom he refers to as "Henry Pym") and speaks about him pretty fondly. Apparently in the original script, there was to be a prologue featuring Hank Pym as Ant-Man in the 1960s, reminiscent of his adventures in Tales to Astonish. But as for a main character, Wright just preferred to use Scott Lang.