I will agree that I would like to see more champions played, but in order to have solid reliable gameplay, strong players often have smaller champion pools.
Personally, I want to be a good 'all-rounder' but I am not anywhere near as competitive as most people who play this game. I'd rather be able to play everyone well enough to be decent than to play a few champions perfectly.
However, I completely understand that a 'one-trick' would most likely destroy me if they have played their smaller selection of champions in even half the number of games I have played my current 123 champion roster.
Riot may care more if we are entertained, but the players care more that they win. They are going to play the champions that they are most comfortable and most consistent with. Also remember that they are not just playing a champion they are comfortable with, they are playing in a team they are most comfortable with as well. They don't have to only worry about being able to effectively set up their own skills, but setting up coordinated attacks with their team. Knowing approximately how often their jungle can clear camps, when they gank, when their mid can roam, when each respective ally reaches their power spikes, ability ranges, cooldowns, ....
And as Trolo pointed out, the champion pool is a decent size this year. It has actually gone up significantly each year over the previous 3 year (I believe that is the case, .. don't quote me on this because I don't recall my source, but I do remember seeing this somewhere. I'm mostly not sure about the number of years. The fact that it has been trending up is a fact though.)