because it is the most hyped platform and because of that there are services like myetherwallet for example that made it easy for people to have an ethereum web wallet despite having a real desktop wallet being near impossible because of the bloated blockchain. and that makes it easier for anyone to join these ICOs.

there are lots of them!
- Waves,
- LSK,
- NEO,
- ARK,
- Ardor,
- Bitshares,
- Counterparty (XCP),
- Ethereum Classic (ETC),
- Expanse (EXP),
- NEM,
- NXT (NXT),
- Stratis (STRAT),
- Tezos

and finally Rootstock (RSK) which works on top of bitcoin