A huge number of new startups showed negative dynamics in the price of the token after entering the market. Therefore, people do not want to buy them during the initial sale. Cheaper to buy on the exchange
Not all the coins are cheaper on the exchange because I have seen many coins/tokens got pumped once it hit exchanges. This may be the reason why people are preferring to contribute into ICOs. There are many exceptions and sometimes you also must be true. It means we cannot have one general rule to invest and when to wait. We need to diversify that is the only solution, it seems.