Because Wizards are just too strong for anything short of a Wizard-level campaign. So, for us who want to play at rogue level, we have to nerf them. But where to begin and when to end? Hopefully this will help.
This is NOT a guide. This is NOT a full run-down of how to nerf dem wizzies. This is a compilation of other people's ideas on how to better manage wizards and bring them down to an acceptable level. The rules here are variants I've come across, thought or were suggested to me. Therefore, they are not to be implemented all at the same time. Individual DMs should see them, read them and then decide which ones they are going to follow through.
Contributed ideas by Ghostwheel (if both are implemented, Wizards get nerfed to about rogue level):
Spell recharge time: Add a small interval to how often a wizard can launch his spells. Starts out with an uncomfortable 1D4+1 rounds, but then reduces it as the caster gains levels. Using a spell actually prevents casting any spell of that spell level and possibly disables it for a bit longer if it has an added specific recharge time. Really nice and logical variant, but allows wizards, and any other spellcaster really, to never be without spells as spells per day are ignored. Sorcerers and bards get even faster recharges, which should fit them. Biggest problem? No war through attrition. Almost (if not) all combat-focused spells have a general recharge time, including Finger of Death, Phantasmal Killer, Power Word:Kill and Glitterdust. Of course, a DM could easily add a non-general recharge time to these spells, limiting their use. Personally, I'd suggest 24 hours. Forces wizards to actually deal damage rather than wait their opponents to miss a save.
Longer casting time: Magic should't be unleashed in a word or wave of the caster's middle finger. This variant forces spellcasters to predict where their opponents are going to be by the time their spell is cast. Standard action spells become full-round and everything else gets even longer. Instant casts become standard actions and immidiate action spells use up a standard action from your next turn. Real nice variant, I like it. Makes Dispel actually matter. There are some spells (like Dispel Magic) that can be used in less time than a full-round action, but you better read that part on your own.
E20 variant: Woo, someone managed to do away with that thrice-bedamned epic level stuff! Hooray! So, no more leveling past 20, but for every 20k experience, you can buy an extra feat. Throw in some Cosmic Destinies to beef up the players even more as they close in on the end of the campaign and you get a very nice set of abilities that make your characters epic without using a retarded set of rules.
Save Or Sucks: Add a penalty to the caster once the spell is complete, but make sure its according to the spell's effect. So, for Power Word:Kill, the caster becomes mute for a short time, maximum 24 hours (or until he rests or for the encounter if that's how you do things). A Phantasmal Killer/Weird spell would cause progressively stronger hallucinations, noone said that jumping into people's minds and drawing out their deepest fears is a pleasant thing. My personal favourite is Finger of Death; the caster's pointer finger rots and falls off, to not regrow (or allow subsequent casts of that spell via the same hand) for a short while, maximum being a day. Good way to irritate your wizards, but kinda makes sense to me. If you use extremely powerfull spells, there's bound to be some sort of side-effect right? It could also be implemented for other spells, Burning Hands for example causing small, harmless tongues of fire to persist on the caster's hands.
Wound Points: So, why would I add this here? Unearthed Arcana's Vitality and Wound points is a very nice system. Initially it just adds a bit of health to characters, but critical hits become all that more terrifying. A single critical hit could fell even an extremely powerfull warrior, much less a mage. Considering that spells have a critical of 20/x2, they only threaten a critical at a natural 20, which is about 5% of the time theoretically (practically, I think the dice are fixed to only roll 20s for the villains). A fighter on the other hand, with say a longsword and improved critical, would threaten a critical at 17-20. That's four times more chances of it happening. So, what do criticals do? They ignore Hit Points and strike at the character's Wound Points. directly. If they deplete the character's Wound points, which by the way are only equal to the Constitution score of said character, then bye-bye.
Alternate iterative attacks: Another great example of how to help all fighter classes in order to mitigate the Wizard's (or sorcerer's or clerics or... well you know the drill). From it, you may be reduced to fewer attacks per round, but they are a lot more accurate. Also makes two-weapon fighting even more appealing. In another variant I came across, there's a small alteration. Instead of only two attacks, you get the full normal amount of attacks. The kicker? You don't apply the -5 Base Attack Bonus for every extra attack roll. So, a level 20 fighter ends up having a full attack of +20/+15/+15/+15.
Mana-based spellcasting: Oh, one of my favourites. I originally loved Unearthed Arcana's spell point system, but that's just because it gave spontaneity to wizards. Unfortunately it didn't really work out that well. The mana-based spellcasting system on the other hand fixes that. It allows for a lot of low-power spellcasting, while allowing the caster to keep his/her more powerfull spells in reserve, forcing the character to think before casting. In conjunction with Ghostwheel's ideas it should lower wizards to rogue, or even fighter level, at least I think so.
Well, that's it for now, I'll add more as I think, or find, them. Any ideas, don't hesitate to put them up.
Again, I stress the fact that these are not to be taken as they are. If, as a DM, feel that you should weaken casters in your campaign, then read these and take your choice on how much or how little you want to do it. Additionally, this doesn't mean taht because you nerfed wizards you should buff everyone else, because then you'd make wizards too weak. Just choose the variants you believe would best suit your world and adjust everything by them.