Because there are no good ones, or because you can't stand anything making things easier?
Uhm. That's a good question. Strictly speaking, normal Arkham is a little too easy for me (one of the reasons why most of the Heralds I create are really tough guys), so things made easier aren't that much appealing to me. But, if the design of Guardians were clever, I'd probably play sometimes with them. Among the official ones, Nodens is okayish, but unless I'm desperate, I don't shop for Blessing, so, even if in play, I rarely use the Blessing deck. Hypnos is broken: flooding the board with clues all the time makes the game so easy that I lose interest very quickly. Bast has a great theme, and some interesting ideas, but the mechanics triggering the Beloved of Bast acquisition are so bad that it never enters play.
Amikezor created some years ago a very good Guardian called Hermes, that basically gave new opportunities to investigators for triggering certain special cards. For example, you were allowed to discard a Weapon while at the Woods to get a Sheldon Gang Membership, or discard a Unique Item at the STL to get a Membership. In this way, the game balance wasn't changed that much, and you were allowed to play with cards that normally never enters play (like Condition cards from Dunwich) or simply give more realism to some other points (Hermes allows you to buy a Rail Pass at the Train Station, that, if you think of, should be always granted as in real life)
Finally, I guess I don't like that much the idea of Guardians because it seems to me like they are more on the Derleth's view of the Mythos rather than on HPL's, but still, probably that's the less important point of those I mentioned