the same reasons why people dont typically like to use cheap yoyos (with exceptions of course). it's just not as stable, and its size is smaller than what most people find comfortable- 5 inches in diameter or greater. The thing about diabolo is that there is not really an equipment gap (ie using an expensive diabolo from the start doesn't really hurt you as much as it would in yoyo).
Investing in a nicer diabolo from the start is a good idea. Also, getting nicer sticks really helps. My diabolo play really improved when I invested in some Henry Carbon fiber sticks.