First of all, the main purpose of Ethereum is for smart contract platforms and therefore its exchange prices and acceptability can depend heavily upon the success of various ICOs and IEOs. The amazing success of the ICOs during 2017 took the Ether exchange rates from less than $10 per coin to around $1,500 per coin (an increase of 150x). But as the ICO market disintegrated, the Ether exchange rates also crashed.
It's not only the struggling ICO-market that affects ETH. Also the successful ICOs of 2017 are a reason why ETH declined. The projects back then raised hundreds of thousand of ether and the ICO-projects converted all these ETH to fiat.