Ethereum was extremely famous in 2017 which prompted an enormous increment in price from simple single digits to $1400 with lots of purchasing pressure from a retail crowd pushing the cryptocurrency high as can be. [/b]
AFAIK the all-time high for Ethereum wasn't reached during 2017 but it was during the January 2018's first few weeks. That made a significant pump for most altcoins and Ethereum took the spotlight.
This happened after bitcoin reaching its all time high so that's possible to happen again when we see a bull run. It will be for bitcoin first then the altcoins will be next. Those analysts that says we may never see it again, okay that's fine.