User blog comment:Lukeatlook/Pyro's Item Overhaul 5 - Supports and the CDR Overload/@comment-5266525-20140719194404/@comment-1330314-20140719204255

Basic attacks and spells don't work the same way. On every hit, basic attacks just apply damage, and sometimes a healing and/or a slow depending on which champion you are and what items you have. Spells, on the other hand, can apply powerful effects, including hard CC. If you were to increase the CDR cap, supports would just be forced to itemize for CDR even more simply because they'd then abuse their even lower cooldowns to spam CC with no consideration for other stats. Diminishing returns would still not prevent this from happening, as supports would be able to abuse the lack of a hard cap to min-max with respect to CDR and output tons of utility and CC on a very low cooldown.

The other issue with raising the CDR cap is that it would create seriously weird edge cases: with just one more point of CDR, could shield two targets at once, and with a little under 45% CDR  could have two Tibbers up at once. Obviously, this would be fixable by either raising the abilities' cooldowns or reducing the duration of their effects, but in the end you're going to have to rebalance every spell around this new cap, which would also limit the cooldowns and/or effect durations of the spells you'd have in the future.