Most of it is the cost/ramp time for a VN. No matter how creative you are, you're going to be limited by your hardware and your knowledge of the physics of rendering programs. That's where a large amount of the bottleneck lies for people who are actually creative, and the fact that you can't just write/draw/record what you want and have it instantaneously come to life can be difficult for some people to deal with. That's part of why you see people who have written hundreds or even thousands of pages of smut/literotica not hack it in the VN space.
Also some people can't script well. It's actually very easy compared to programming... I've debugged hundreds of VN releases just for my personal use, and submitted some fixes to devs or to forums. But for some reason, some people have a hard time with Ren'Py scripting despite how exhaustive the documentation for it is and how simple it is relative to other types of scripting programs. Also some people go way overboard and try to do actual Python programming when they only need the tools available with basic Ren'py scripts... if you know what you're doing, that's fine, but most of the time they hire some Russian college student or copy templates that they don't know how to adapt for their purposes and just end up with a buggy mess that they can't maintain.
The point being, it's a multidisciplinary endeavor, with high upfront costs (at least if you want to create high quality content) ramp-up time, and low potential to recoup the costs, especially if you're doing content that the payment processors are gonna crack down on and the SJWs are gonna come after. You truly have to be in it for the love of the game.