1) Food supply. Trolls, dragons, giants, remorhaz, and such need a WHOLE FREAKING LOT of food. Without people and animals making up a majority of the population, these big nasties would quickly starve to death or resort to eating each other, which would diminish their numbers and make it easier for hidden enclaves and other critters underfoot to expand and get strong enough in numbers to overwhelm some of the big nasties.
2) Demons and devils are rivals, and interfere with one another (to say the least), while celestials try to interfere with both and other fiends. Plus they usually need to be summoned/called by folks in the Material Plane. And they don't generally have class levels, so not as much high-end spellcasting. Other monsters compete as well.
3) Class levels. Monsters need time to grow into their power. Humanoids multiply rapidly and develop in classes that their kind have developeda nd honed over the generations. By the time a dragon is powerful enough to cast 1st-level spells, a human born in the same year may already be an archmage hurling Meteor Swarms and Wishes to and fro. By the time a frost giant is big enough to crush humans like gnats, a human will have already become a weapons master or master assassin, and will cripple the giant in seconds and slay it swiftly.
4) Deities. Humanoids have by far the largest pantheons around. The Great Mother and her small number of beholders (who have extreme rivalries and enmities among themselves) can't possibly interfere with the ENTIRE Baklunish pantheon (let alone the Suloise, Oeridian, Flan, and Olman pantheons, and that's only covering the Material Plane of Oerth) helping out their human worshippers to thrive despite the threat of a few measely beholders that can't even stand to work together. Sekolah can't help her sahuagin take over the seas and the surface while competing with Blibdoolpoolp's kuo-toa and Panzuriel's krakens and such, at the same time as they try to deal with Eadro and Deep Sashelas and their merfolk, aquatic elves, tritons, and such (who are much more apt to work together and with other races).
The giant pantheon has its own internal problems, and giants don't get along so well with different kinds of giant, so they're not such an overwhelming threat to humanoids. Dragons and their pantheon are self-explanatory in their rivalries and inability to conquer humanoids all over, with their own enmity between different dragon breeds. And the goodly or neutral humanoids have more deities on their side than the evil humanoids, generally speaking. Sure, some of the good/neutral humanoids' deities are evil, but even those ones usually want themselves, and their faithful, to be dominant over the monsters and their monstrous deities in the world.
So in short, the numerous humanoid deities (who often, but not always, receive more power from their very-numerous and fervent worshippers than many of the monstrous deities do from their few and less-devout monstrous spawn, though such is not always the case) trump the monstrous deities and thus have more freedom to help their humanoid followers (you'll notice there are many artifacts made by humanoid deities, not so many made by monstrous deities for their worshippers to use). Humanoids have the gods on their side, more often than not. And that means a lot of clerics and such, as well as a lot of wizards and such since humanoids are the ones who most often pursue such paths.
