star wars - How was Grievous able to defeat Jedi?

Since Grievous could not use the Force, wouldn't a simple Force push always eliminate the danger to a Jedi?

Note: unlike many of the other enemies of the Jedi, Grievous appears to have no weapons but his lightsabers, so he has to get up close and personal. (electrostaff is still a melee weapon).

Correction: he had a blaster as well.

Answer

Grievous

General Grievous was, among other things, an expert tactician. His grasp of strategy may not have been excellent, or it may have been hampered by the Separatist's real leadership, but he was indisputably a master tactician.

He was trained in lightsaber and electrostaff combat by some of the best in the Galaxy (notably Darth Tyranus) and his cybernetic body was capable of maneuvers that a typical humanoid couldn't do. It is also possible that his brain (which, if I recall, did have some cybernetic enhancement, and at least had some interface with his cybernetics) was divergent enough from 'standard' to mitigate the Jedi's mental tricks.

In short, he was as fast as a Jedi, he could see and process what he saw as fast as a Jedi, and he was at least as well trained as most Jedi in lightsaber combat.

The Jedi

Your typical Jedi Knight or Master during the Clone Wars era served as a General, or possibly in a support role. Jedi were too rare and valuable to serve as common troops, and there were no specialist Jedi platoons or companies. They typically served as the leadership and left the fighting to their troops. Some Jedi (notably Obi-wan and Anakin) were noted as leading from the front or taking it upon themselves to stage special operations against specific targets.

In an all-out assault, some Jedi would no doubt place themselves on the front lines - Mace Windu in particular was a Force powerhouse, capable of piling droids up like cordwood and knocking common battle droids over in windrows.

What the Clone Wars did not give Jedi much experience at is lightsaber combat. Sure, they primarily used their 'sabers against droids. That said, fighting droids with a lightsaber is primarily a matter of ranged defense. You move fast, using force-enhanced speed, block incoming bolts, close to melee range, and it's done. Even Super Battle Droids are trivial to destroy with a lightsaber - you just cut straight through. They can't block, then can't effectively strike you, and one or two blows will destroy them.

Droidekas, the shielded wheel droids, were a different matter: their shield was apparently proof against lightsabers, as evidenced by experienced combatants not charging at them. In those cases, Jedi typically had to avoid action and let the clones, often with specialized weapons, handle them.

MagnaGuard Droids

These droids, specifically designed by Grievous, were capable of going toe-to-toe with most Jedi. They might win, they might lose, but they could put up a fight. Obi-wan and Anakin both had difficulty dealing with them, and they are noted as some of the more skilled lightsaber combatants in the Order.

Conclusion

General Grievous was immune or resistant to many of the Jedi's common tricks, their equal or superior in single-blade combat, capable of moves Jedi would not easily anticipate or have trained against, and had dozens of tricks up his sleeves.

His training under Dooku would have given him an enormous edge over most Jedi - Dooku, one of the Jedi Order's most notable swordsmen, knew all their tricks and training habits. Grievous' physical attributes were at least the equal of his MagnaGuard's, and he could easily use two (or more) lightsabers.

That last sentence, above, is the final nail in most Jedi's coffins. It was extremely rare for any Jedi to use anything but a single lightsaber. Maul's double-bladed saber, the two-saber style, and other oddities were vanishingly rare. This meant that even an exceptionally skilled duelist would be at a disadvantage: they would be trained to watch two threat sources: their opponent's blade and their opponent's Force abilities. Against two blades, even skilled duelists suffered (reference: Qui-Gonn). Those who were extremely skilled, especially at defense, could handle two blades (or two attackers) somewhat well. But when four blades come into play, controlled by a single mind? Grievous could present dangers from three or four directions at once, and took advantage of this fact.

Against all of that, the ability of a Jedi to use a Force Push (itself an attack which is frequently telegraphed and which seems to be at least moderately tiring to use) to push Grievous back some distance doesn't seem like much of an advantage. Watch him when he battles, you'll see that he almost never puts himself between a Jedi and an environmental hazard (pit, spikes, chemicals or heat) that can hurt him. All a Push will do is tire the Jedi out and put some distance between the combatants.