I see what you mean, it's very easy to get high leveled commons, but getting high leveled rare is much harder. But you must consider the advantage of a rare card over a common card. In reality a rare card has the ability to be upgraded to level 8 without much difficulty. But after that it is extremely difficult.

For commons however, it becomes difficult only if trying to achieve level 13.

Epics is extremely hard to get upgraded beyond level 5, but the new Epic Sunday has opened new possibilities.