A lot of people pushing Blockchain technology have a vested interest in making it a more pervasive technology. Because Blockchain applications drive up the value of their crypto assets and offer more opportunities for speculation using crypto.
Blockchain applications don't necessarily need to use existing cryptocurrencies. However in that case you need to find some other incentive to get people to mine your blockchain, which is hard. And consequently having a small mining pool makes you vulnerable to a 51% attack defeating the entire purpose of a "tamper-proof" log. Thus it's easier to just use Ethereum, which in turn drives up demand for that cryptocurrency.