Why is Riot afraid of promoting specialized champions?

I'm not here arguing that champions don't currently have specific and balanced strengths and weaknesses, they definitely do. But whenever someone a champion or class of champions present a new set of win conditions to the game, Riot seems hell bent on squashing it. This continually pushes those generalist champions to the fore front. If you put together a list of all the things a champion has to be good at within a role, the number of things that has to be checked off is way too high. There at times have been champions who excelled at a few things, but excelled VERY WELL at those few things, but they got squashed.

I'm not going to make a whole list for the sake of brevity, but here are a few that come to mind right away:

• Full poke/siege comps
• Split pushing as a game ending win condition rather than just a means to begin a snowball or trade down
• Pure control style jungling (looking at you Nunu)

Before getting responses I'd like to note that all of these things have been viable int he pro scene at one point or another without being so strong that they were all that way played. Pure control style jungling was viable while Lee Sin and Elise were top tier picks, Poke comps were a thing when team fight wombo was a thing. So if these things are not strong enough to edge out large swaths of champions or styles, why is Riot afraid of letting specialized champions exist in that capacity?

Love to get your thoughts!