Because there were too much of them in 2017. Good projects are already made, junk only left (except some, but they are rare). So, there are less chances to get even a soft cup. Thus ICOs will be less popular in 2018.
So what will be the next big thing?
I believe the next big thing will be trade.io, its already doing so well. Raised $12mil in Pre-ICO and ICO will start tomorrow 7th Dec. They have joined HiTBTC exchange and other goals have been reached also.
For future reference you might want to check brand new ICOs too, check DIW and Wabi too. These look really good too.