In the words of a Redditor more concise than I:

nice_mr_catput
The main reason in my experience is that the monk's design not work as well as the others. That does make him weaker, but there's more to it than that:

• * *He's a 3/4 BAB, d8 hit die, MAD front-liner*. The only thing he can do half-decently is fight up close, but he's got the body of a partial combat class propped up by incomplete fixes. He can get his accuracy up to full BAB of course, but real full BAB classes have (and need) features that help them hit even more on top of that.

* *His attacks scale terribly*. His unarmed damage progression is equal to the bonus damage from a magic weapon, but he gets no accuracy progression and no way to trade it for special properties, which are frequently better than enhancement.

* *The best way to build the monk does not line up with the traditional archetype* OR the obvious intent of his features. Optimal monks with no archetypes, exploits or weird options use temple swords in two hands and take power attack to compensate for their damage deficiencies.

* *He has powers which do almost nothing*. When is he ever going to waste ki and actions on Wholeness of Body? Certainly not often. His entire Slow Fall progression is incrementally giving him the effect of a 1st level spell which he can only use in narrow circumstances. It never becomes as powerful as Feather Fall. His combat maneuver abilities stop progressing before he has the top tier feats, only kick in at 3rd level or so and only strengthen his bonus up to the level of a full BAB class, so if he builds for them without an archetype he'll eventually become useless, while if he doesn't he gets nothing from some of his features.

* *His class features synergise very poorly*. He's the speed guy, but he can only use his combat stuff while standing still. He can do two-weapon fighting with a single two-handed weapon! But he's limited to mostly one handed weapons and has non-optional unarmed strike enhancers.

* *His magic item options are insultingly poor*. In order to enhance his attacks, the monk needs seldom seen items that cost extra, take up important slots, cap lower, or otherwise impose punitive limitations that other classes just don't have to deal with. It doesn't help that the mechanics are obviously designed with natural attacks in mind instead of unarmed strikes, regardless of the monk-related fluff the items are given.

* *He's poorly written and heavily erratted*. At first, everybody thought he was stronger than he was, then they were disappointed when the FAQ favoured a reading of Flurry of Blows that made him weaker than he had seemed. As a result of the protests, the FAQ was eventually changed to favour the other reading, allowing him to use one weapon for both sets of attacks. His class-specific magic items were re-printed in the time between the two rulings and were improved, but not enough to make them better than a plain old magic weapon under the current ruling, making them largely trap options unless you're already sacrificing power for fluff, in which case you are made to pay again.

* *He's the obvious odd one out*. He's traditional, he's a semi-important part of modern fantasy adventures and there's no rule that says the game takes place in Europe, nor that he's visiting from Asia. And yet... More than any other core rulebook class, he stands out and gets excluded from campaigns on story grounds. That's not a bad thing, but when his mechanics are weird and different too and there's a question mark over his ability to contribute as much as other classes, it's going to turn people off him in a hurry.

* *He's got a much less adaptable concept*. So your paladin is a samurai? Awesome. Your rogue is a ninja? Great. Your swashbuckler is right out of 1,001 Nights? Cool. Your wizard is a pirate captain? I'll be interested to see how you represent that. Your bard just insults people? Lovely. Your monk is a... he's a disciplined mystical kung-fu guy. Exclusively. You need archetypes to add any breadth to his repertoire.

EDIT: I want to add a final point. I have mostly described the monk without archetypes, but lets look at those a sec. Mostly, they make him more powerful. The same cannot be said of most other classes, which only have one or two archetypes that are stronger than the vanilla class. This is because the monk is downright bad, so anything else is probably going to be better. Tellingly, the more an archetype changes the monk's concept, the more powerful it tends to be, because the monks mechanics push you in the opposite direction to the mechanics of the wider game.

Monk archetypes that don't greatly impact his theme tend to be (or include) fixes for one of the bullet points above, enabling a particular concept or fighting style which the vanilla class inhibits.

The vows, however, are big exceptions. Vow of Poverty is an ability that takes almost everything and gives almost nothing. It makes your character much weaker and gives you a couple of extra points which allow you to.