1. English speaking peoples generally choose not to show any undue signs of respect for Kaiser Wilhelm. I'd be one of them. Indeed, calling him "Kaiser" is the height of politeness given that I'd prefer to just refer to him as that fat old arrogant German bastard.

2. Emperor in English means one is above the rank of King and has Kings who pay hommage. Given that Britain is a monarchy, to assert that Kaiser Wilhelm is an Emperor is to assert that he outranks the British monarch. This is categorically rejected by English speaking peoples.

Indeed, Kaiser is close enough to Caesar for it to sound almost insulting. This is appropriate.

Btw, the Czars of Imperial Russia are always referred to as "Czar" and never by any other title. Same reasons.