There are a number of reasons and have been stated earlier in the thread, but I'll give you a few:
1) first and foremost sm divers on boats are more of a pain in the ass than bm divers. SM hardcores (which I have been for many years) will argue that that's not true. BS. It takes more space on the boat and mroe time for a good sm diver to get in the water. Add to that, imo and many boat captains' opinions, climbing a ladder in pitching seas with doubles is safer than sm. If a sm diver isn't pulling up to the side of the boat and hanging tanks, then typically they are climbing the ladder with both tanks on. A very good number even unhook bungies first. In a pitching boat those tanks are not solidly on your sides like doubles are on your back, making for a potentially sketchy exit
2) Alot of boats are banning sm
3) SM is very finicky. Even if you take a good quality sm course, sm is always going to be more finicky and fidgety with gear. For a newer, nontechnical diver there is the potential for a steep learning curve
4) There are alot of shi--y sm divers nowadays because it's become mainstream. A big portion of this is due to the fact that many OW divers jumping into sm barely have their trim and buoyancy under control. Throw in sm, and now you've got a cluster-f in the water. I've seen it many times, even in the caves where most people go in with decent skills
5) There are alot of shi--y sm instructors now putting out shi--y students because it's a cool money making fad.
6) Sm is more of a unitasker. If you're buying a sm setup, that's all you'll be using it for. If you buy a bp/w, you can dive singles and doubles with it (may just require a different wing size)
7) SM feels great in the water, but so does bm if you're properly trained or properly capable using doubles. In the water if you're not doing tight penetration dives, there is no benefit to sm over bm. Many sm divers will say they feel much better in sm than bm. I was that way for a long time, but I eventually re-evaluated my weaknesses in bm and realized it was me not the system.
8) As you move into the tech world, stage and deco bottle management is not as simple as bm. I can very easily dive 2 stages and a deco bottle in sm. It took alot of tweaking and modifying how I carried and setup the stage to get it right. It feels very good now, but it was work and it is for many people. There's about 50 different ways to carry and setup a stage for sm. Really only 1 or 2 typical ways in bm. That means in the future if you make other technical buddies, it's easier to both show up to a dive site and have somewhat similar gear setups. Less time explaining to your buddy how to address emergencies in sm if they occur if the buddy isn't familiar.
Hopefully that's a good enough bunch of reasons. I have more, but don't feel like continuing, especially because I know I'm already going to get the same couple people arguing the same points they always do. And that gets old.
And one last time: I am a very big sm proponent and have many years and 100s of technical dives under my belt in sm, and I really enjoy it. But at the end of the day if I just want to go dive and have fun, bm doubles is the way to go. Gear is setup and ready to go out of the back of my car. Just put it on and go. No fiddling or faddling around with stuff.
Learn bm first and the principles necessary to dive a redundant system (valve checks, s drills, emergency management, etc). If you find a point in your diving that sm is necessary, you're transition over to it should be very easy. SM was invented to be a tool, but it's been adopted as a fad and from what I've seen I don't think that's a good thing.
Disagree completely with your assertion. Sidemount is a perfectly good system for general purpose diving and is far superior than others for ease of use and redundancy.
I'm not surprised. You don't seem to agree with alot I say and vice versa. I have no interest in trying to argue points with you because it historically is a waste of my time