Because pulling off that gameplay in a modern setting would be a *lot* more work.
Historical settings have the benefit of being smaller in scale and heights, and focused more on melee and fisticuffs than firearms.
What? It wouldn't be any more work. There's no reason at all they can't make modern games that don't focus on firearms, games do that all the time. You can literally just change the art to be 'modern' and make the exact same game.
I think it's more fans just wanted the past so the games moves to focus on that despite the series starting with the goal of focusing more and more on the present day. That was whole point of Desmond and the bleeding effects, to eventually have a mainly present day AC game
The movie doesnt have to make fans happy for 15 hours and thus can focus on what the story was originally about: a sci-fi story where the exploration of the past is crucial to saving the future
Claiming the 'original intent' was to move to present day means literally nothing when the series never went that direction. This isn't a movie from some alternate universe where that happened, this is from our universe where the present day stuff is virtually meaningless.