There's a bunch of coins (largest being etherium) using a proof of work mechanism which is optimised for GPUs and can't really be more efficiently implemented by an ASIC (rough reason being it's limited by memory bandwidth, not hashing speed, and GPUs are basically already super optimised for memory bandwidth). This was a deliberate choice by the designers of the cryptocurrency because ASICs centralise mining even further.

Only real reason to mention bitcoin is that basically all cryptocurrencies go up and down in price with bitcoin, so when bitcoin goes up in price, other cryptocurrencies tend to do so as well, incentivising mining all of them.

ASICs are not that easy to come by and they cost a lot. Newcomers to mining use whatever they can get their hands on.

Depends on the cost of the coin. BTC has been climbing.