For whatever reasons I'm attracted to things that are a bit not politically correct. In my shenanigans, I've read a lot of controversial opinions about WW2 and how Hitler wasn't the bad guy, for many reasons but the crux of it sometimes comes down to the military strategies.
Its a tricky question and as far as I know there is no agreement about the exact reasoning.
First, I think it is clear that he did not want them to escape. He belive it would take them much longer to get away, and he belive befor the could he could destroy them.
I read the book Panzer Leader by General Guderian who was the commander of the tank forces that was spearheading for Dunkirk. He wanted to attack directly, he belive that he could have destroyed the troupes.
He reports that he was order to stop even after him strongly disagreeing and wanting to push foreword. The High Command seams to think that infantery comming down could and should do the task. Also it seams that Göring strongly argued for the Luftwaffe.
Some resonse might be that hitler did not want to risk his tank forces, another reason would be to deprive Guderian of the victory. The reasoning for that would be to not give even more power to the old prussian wehrmacht generals.
A Nazi Historian might chip in and add something I dont know about, because my main source are books by generals.