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Ethereum has a stable price and also supports ERC tokens so it is hassle free for them, however they can use other crypto currencis such as Dash NEO, etc.
The simple answer is money. It just cost too much money to build and maintain blockchain and even big companies like Kik are looking to use existing blockchain like eth (they will migrate to stellar). Tron(trx) is another project with big financial backing which is using the eth network at the moment. Maybe in the future they will build their own blockchain or they will migrate to some of the new faster blockchains but in the beginning it just make too much sense to begin with erc20 token.