こんにちは。I think it's a really good question. I never thought about these differences until I read your question. In my opinion, it's a matter of context(文脈) more than grammar. When you use "deha" there may be something else to compare with. For example if you say "AはBでは人気があります。(A is popular in B.)" you may want to imply that A is NOT popular in somewhere else, let's say "C", or that the fact "A is not poplar in C" is already obvious to the listener. It's not wrong to say "AはBで人気があります。" here but in that case it is simply a statement. I'll list up some examples. Example1: 納豆は日本で人気です。 Natto is poplular in Japan. 納豆は日本では人気です。(他の国では人気ではないかもしれない) Natto is popular in Japan(but may not in other countries). Example2: 映画料金はひとり1500円です。 The movie ticket costs 1,500 yen per person. 映画料金はひとりでは1500円です。 The movie ticket costs 1,500 yen per person(but the price changes if you buy a pair ticket or something). Hope this helps.