Celerity is VERY champion specific.
It only adds an 8% bonus to your bonus movement speed.
Look at it this way, teir 2 boots only gives 45 movement speed, meaning that you're only getting 3.6 bonus movement speed. Of course, 90% of the champions in the game don't even care for movement speed, except champions like Hecarm, Rammus, or something like a roaming support. The only other way to use this bonus would to take ghost, which again is VERY champion specific.
But at this rate, is getting another 12-15 movement speed max worth taking over something like transcendence? This rune is superior in every way, 10% cooldown reduction + bonus adaptive damage for capping over. Why would you not take this over Celerity?
and
items
due to his lack of mobility, but it was almost inconsequential and not worth taking.