99% of them are for greed. People are hoping to cash in on the "easy" money that is crypto currencies
Very few of them offer any innovation or purpose at all. Those are usually the ones that stick around.
The others are pump and dump schemes or scam ICOs designed simply to make money for the creator and the few that are involved at the beginning.
It's so easy to clone a coin now, change a few variables and voila instant new crypto-currency. Also since this is an unregulated area it is prime real estate for scammers and hustlers to come in, create a coin, scam, pump and dump, abandon the coin, create another and repeat over and over again.
Since the coin never actually "dies" (the closest thing to death would be to be delisted) the number of crypto-currencies simply keeps increasing over time