User blog comment:Lukeatlook/Pyro's Item Overhaul 5 - Supports and the CDR Overload/@comment-5266525-20140719194404/@comment-1748472-20140721162532

Because pretty much every champion is balanced around the 40% CDR cap. Give them more and the results would be total chaos. Rebalancing that would be a tremendous amount of work for a small problem.

Also, diminishing returns discourage stacking and burden the player with one more problem ("At which point do I stop buying CDR because the costs outweigh the benefits?"), which would result in almost the same restrictions and more background knowledge clutter. People would eventually search for a "sweet spot" where the cost effectiveness is at its peak and that spot might be different for every champion and depend on the playstyle. And that would pretty much become the new cap ("WTF Y U BUY MORE THAN 60% CDR ON LUX? TOTALLY WASTING GOLD!").

So it would hardly solve the problem but simply change its form and make the whole matter a whole lot more complicated.