The way I see it is there's a lot of info on how to clone coins but very little on supporting them. Cloning is the easy part.

Getting updates working on existing blockchains, doing successful hard forks and being able to regularly compile updated working wallets on the main O/S's separates the men from the boys.

Too many devs give up at the first hurdle as soon as their coins fall victim to 'difficulty issues' i.e. pool-hopping or they realize that getting on exchanges is a lot harder than they imagined. Also, keeping miners interested enough to keep the blockchain ticking over could be another reason so many give up.