Why doesn't 121 gear up? Simple:
Awesome warning systems
Even in old-ass airplanes they have better warning systems than in the piston twin and light turbine twin world. Look at the navajo for example, if I remember right, the gear warning horn is supposed to sound at 18"MP if you have it set to the right settings. That means that the gear horn sounds in the flare! I've NEVER heard the gear horn in the navajo except in training. If you tweaked that up to 23"MP, you'd eliminate a lot of the gear ups right there because if you could even physically fly the approach, you'd get a gear warning. "TOO LOW GEAR" and other aural warning systems are the law of the land in 121, not the exception. Not to mention that disabling these systems is VERBOTEN. The Private Otter that Ted Stevens hit a mountain with? TAWS was disabled.
<side rant>Hey other Alaska guys! Don't disable warning systems that are going to save your ass unless you're in awesome VFR. TAWS, EGPWS, and the Garmin terrain alerts are trying to keep you from getting killed, they shouldn't be turned off unless hitting something is definitely not going to happen, or you're going to an airport that isn't in the database and you can see that you're going to land there. I personally don't disable those systems unless I can see better than a couple minutes of flying time in front of me - that could be 8NM or 3 depending on the airplane and how slow I'm going.</side rant>
Good training and good procedures
Generally speaking, unless you go to FSI or Simiflight or whatever, in house training in the 135 world is of dubious efficacy at most places (there are exceptions! put down the pitchforks, even where I'm at now I think has good training). Training at a lot of places is "read the book, checkride's on wednesday morning, training flights on monday and tuesday should iron the bugs out good luck. Procedures often times aren't standardized, or if they are, they may not be that good. 121 has whole departments of people thinking about how they train, and how they should fly, it's not one guy who's trying to be chief pilot and fly the line who's doing it.
<soap box>As for procedures - whatever it is that you do, do things the same way, in the same places, every time. For example, "the gear go down when I'm abeam the numbers, or 1NM before the FAF, or Glide Slope Intercept, or Intercepting the VASI, EVERY TIME." Here's another piece of good advice someone gave me, "check the gear are down when you're pretty much sure you're going to land." That could be at 100', over the fence, short final, whatever. But check, check, and recheck, and always do it at the same freaking time. I do this so frequently, that on days where I fly the Caravan, I have looked down for gear lights as I'm coming over the fence and has a moment of panic "CRAP I LEFT THE GEAR UP! OH wait...I'm in the caravan." That's not a bad thing, gear position is about the only thing you can royally screw up on most light airplanes. Another one, after you select the gear down, keep your hand on the selector until you get all three green - that way a burned out light doesn't screw you over.</soap box>
Stabilized Approaches
Put down the pitch forks 135 brothers, but it's true. I have seen (and flown) many an unstable approach in the 135 world, and we wonder why they don't work out well at the end. While doing barber pole to 3 mile final, then chopping the power and pushing the props forward may be fun, those late term configuration changes sometimes get dicked up, or you get distracted, and then an airplane gets totaled if you're having a really bad day. If you want to avoid gear ups, all configuration changes maybe ought be done no later 1000' IFR and 500' VFR (that advice is worth what you paid for it). That'll give you time to think before you gear up the airplane
It's glaringly obvious if you've forgotten the gear if you're stabilized for awhile, "man, I have the power way far back, this is weird...HOLY CRAP GEAR!" Every 121 airplane out there these days has to be stabilized otherwise FOQA knows before you've gotten to the hotel. Do some of your own quality assurance after every flight, and you may realize that "hey, I was so tired that I flew that pattern without doing a gear check. Or, "man, that approach wasn't really stabilized, that could have been better." This is easier said than done, I know, but I think a self-reflective pilot is a pilot who learns is a better pilot. We all have our days where we just want to get the bitch to the barn, but take a little time to debrief yourself when you're done, I try to (with some success) and I think it allows me to be aware of things I can focus on to get better at what I do.
That's enough ranting for now, but I'll leave one thing - the least affective risk mitigation strategy is training, the most effective is to engineer the problem out of existence. You can more or less do that with gear-ups with all the warning systems we have available today.