Reason number one:For pump&dump purposes.
Reason number two:Exit scam ICO(which is kinda like the first one,but with pump&dump they could pump the coin again and gain aditional profits).
Those are the real reasons.The fake reasons include:instant and cheaper transactions,easy and profitable mining,technology innovation that will beat the crypto competition.
It's unfortunate but this is the truth, on the surface there are many projects created with the hope of improving cryptocurrencies in general or a particular aspect of the blockchain, but most of those projects are nothing more than an attempt to earn money by the developers, if you look at the market you will see that people are losing faith in icos and when you think about it this makes sense.