Droids are expensive, and these droids are very expensive. They carry four guns and personal shield generators. The Trade Federation could probably make many B-1 droids instead of just one droideka.
2). Combat effectiveness
Droidekas are clearly very powerful. Their shields can take a lot of damage before losing integrity. A strong blaster bolt (or a lightsaber to the top of their head) from some kind of vehicle is required to destroy one. That being said, they don't really seem to be strong in numbers unless they're lined up. B-1 battle droids seem to know how to shoot around their fellow troops, but droidekas have massive shields that extend off their bodies. This makes it difficult and dangerous to shoot around. Droidekas could become very dangerous to the surrounding battle droids because of their deflector shields. It's pretty easy for me to imagine one droideka accidentally shooting another and the shot ricocheting and hitting a battle droid in the face. Meaning that it is far easier to just use a bunch of battle droids. This, after all, is the Trade Federation's battle strategy; use a large amount of not-very-powerful troops to attack instead of a few strong ones.
Droidekas are not smart at all. They are killing machines that can't communicate (at least verbally) to the other droids, unlike B-1 battle droids. They can't give orders or make tactical decisions regarding their environment. Granted, B-1 droids are pretty dumb too, but they can communicate with other droids and give orders. In addition, battle droids can pilot or drive vehicles when needed, something that droidekas can't do.
So why doesn't the Trade Federation employ more droidekas? Because…
Thery're expensive to produce
You can't put that many on the battle field without risking B-1 battle droids, and it's not the Trade Federation's style
They're stupid and can't do anything more than shoot at stuff (and roll around).