The video game industry is a strange and fickle beast: sometimes you'll wait a decade-and-a-half for a much-demanded sequel to finally be confirmed (Shenmue III), while elsewhere a sequel almost nobody actually wanted is greenlit in no time (Knack II... which actually turned out pretty well in the end).
Sequels keep the gaming world spinning, with many publishers imposing restrictions on the developers they work with, that a certain percentage of their output must be entries into an established franchise rather than a gamble on a new IP. It's cynical as hell, but it works.
Still, that doesn't explain how games as beloved as these 14 titles, many of them significant financial successes, haven't been able to secure sequels yet, and almost certainly won't ever do so.
Sure, studios can only allocate so many resources to making games, but given how much these games matter to so many, it's nothing short of criminal that fans are going to be deprived of yet more adventures in the same vein...