Why did ICO suddenly become so popular?
  There is no unambiguous answer, but there is something that brings us closer to the truth:

1. ICO is much cheaper than IPO (and safer, but not for investors, but for
organizers). For example, if you are organizing an initial placement
thousand (dollars, of course).
2. ICO organize simply and from a technical (organizational) position:
on Waves, Bitshares, Ethereum ... and a number of other services, like, say,
https://www.walletbuilders.com/.
3. ICO is not yet regulated by strict, mandatory norms,
as it is in the banking sphere or in the field of donations,
so it is easier for depositors to enter, and for investors to create.
4. ICO, among other things, is a technological process, and IT - always attracts a diverse audience
(example iPhone - perhaps the best in the new world).
You can, for certain, fetch and even more reasons.