Why make this like a theatre play?

What I think is probably the most interesting part of this movie (and what eventually makes this a must-watch in the end) is that this feels more like a play than a movie narrative. Not only the setting (with its limited space and deliberately fake looking set), but also speech patterns and speech flows of the characters as well as certain scenes having aspects of interludes, definitely remind of other plays. I wonder whether Lee sees this in the tradition of African-American theatre. It would be interesting if this would have been a play from the beginning. I know that Lee likes to tell his stories like this. Malcolm X has a similar feel to it. But, why make a movie in the end, wouldn't a play be as powerful?