My opinion is that at the time of ethereum ICO there were not many tokens to invest your bitcoins on, and see the trend now a days. A lot of ICO every day pops up on bitcointalk.
And also the price of bitcoin is bullish now a days so people don't want to invest their bitcoins anywhere else.
There are many more complex factors than that. Ethereum was very promising even back then. If there was a similar market around Ethereum back then as we have now in the ICOs, I am sure they could have raised a few hundred million dollars at the very least.