There are many ICOs and tokens on other platforms, including Bitcoin. E.g. [see here](https://coinmarketcap.com/tokens/). #3 and #4 are on Omni, which is Bitcoin-based.
Ethereum is actually technically inferior as a platform for tokens, but it has a fantastic community which is very much into tokens, so that's why it is much bigger than what you see on other platforms (BitShares, Nxt, Omni, Counterparty, WAVES...)
Ethereum-based tokens have grown to 85% of the total token market cap. Most if not all of the applications built around Ethereum's ERC20 standard, like Etherdelta and 0x, are possible because the EVM enables Turing Completeness. That makes Ethereum far superior to the other platforms mentioned as a token platform.
