I'm going to give you a much longer answer than you wanted, but that's how I roll. Strap in.
1.) They got there first. Being the first one to really nail a growing market is a wonderful thing. For a long time bass players used guitar amps, or small combos. Ampeg was one of the first amp companies to really focus on bass players, and when loud rock and roll became a big deal, their 300w tube head and 8x10 cab was one of the few things on the market to compete with 100w guitar heads and dual 4x12 stacks.
Look at Levi's jeans. They established themselves as the denim brand in the 70s and 80s, and even though Arizona, Old Navy, Lee, Wrangler, etc offer the exact same product at a quarter of the price, they remain a dominant brand that still demands a high price. They sort of invented denim jeans, so it is hard to dislodge them from their position.
2.) Icon Status. Once the SVT started dominating the market, it created a feedback loop. The MI world is very unique in that what your favorite players use has a massive impact on what you want gear wise. "I like how they sound, I want to sound like them, I need the same gear." The SVT-CL is a beast of amp and far from cheap, so there are a few reasons to avoid them. However, in reality there are only so many ways to achieve 300w in a tube amp, and the tone stacks generally aren't too sophisticated. So, company B might make something a lot like a SVT-CL, but my favorite player uses an SVT-CL, and the competitor isn't that much different. Guess which I'm going to buy?
3.) Nostalgia. In case you haven't been paying attention, guitar\bass\drum driven rock is being pushed to the margins and in a death spiral financially. This impacts what people want, and what MI stores will carry, in subtle ways. For older players, they want to harken back to the era they remember; one without all this electronic stuff. It is safe and comfortable. For younger players, they might romanticize a period before the music industry strangled the last few nickels out of artists and artistic ability faceplanted. Given modern advances in recording software, class D amps, and sound systems in venues, there is little reason for a 300w tube bass amp; let alone one that weighs as much as small adult human. The attachment is emotional, not logical.
For MI stores, it is purely economics. They have had to reduce their foot prints and inventory, so they are going to carry items that will sell. The SVT-CL is a proven seller due to its status and iconic glamour. If a store is going to carry a high end tube head, that is the one they are going to carry. Which means if you go to your local MI store to buy a tube head, they will probably have a SVT-CL. Because people buy that amp due to availability, audiences see it on stage, think "that must be the best," and go buy one. Rise, repeat.
4.) They are reliable. I have my thoughts on Ampeg's SS gear, but for all of their other shortcomings, Ampeg's tube amps are very reliable. You see old ones still puttering around, and you new ones getting miles put on them. They are an absolute nightmare to deal with, but very solid (literally and figuratively) pieces of equipment.
As others have said, I would be pretty happy with an SVT-CL + 810 in a backline or studio. I don't see many situations where I would own either. If I'm gigging, the last thing I want to do it move 300lbs of gear on and off stage for a 45 minute set that probably doesn't pay. If I'm not gigging, what the heck do I need 300w for? A venue that doesn't have a full PA is probably too small for 300w, and a larger one will have a PA, relegating your amp to a very heavy monitor. But that's me. I'm always looking to work smarter, not harder.