The main reason of ICO investors are decreasing is due to lot of ICO are scam and even if the ICO is gone good they are taking lot of time to release the coin and list in the exchange. That is why it is better to buy in the market when the ICO coin is listed.
At the moment i also believe that these are the main reasons why there are fewer and fewer investors in ICOs. All the scam ICOs have almost destroyed the market and driven away many investors. In addition, many ICO projects have their token prices crash after the ICO and do not offer any purchase support for their own token. Investors also know this and prefer to buy their tokens after an ICO if they are cheaper. Therefore i do not think that we will see many ICOs for a long time to come.
You put together a strong argument that sounds very logically. Yet the numbers point toward a different reality, the amount of ICO investors or capital that is invested is actually increasing. I agree that this makes no sense at all. Still, this is the data.