Why is the answer לקוף instead of הקוף?
The word מתחת is followed by ל. Just the way it is.
The word is "from under" followed by "to" whatever.
There is a banana (from) underneath (to) the monkey.
ha-banána nimtzét mitáchat la-kof.
I can't see the logic behind this
Yes, prepositions are difficult in any language acquisition. Thanks for pointing out the distinction. Modern Hebrew definitely inherited the idiomatic usages of these prepositions, because the tendency you point out is in classical Hebrew.
Why cant we say: הבננה נמצאת תחת מקוף
מתחת is follwed by ל