Rhetorical question: Because they all value form over function.
I'm so tired of every vtt system I find trying to crash my computer because they all want to have 16k resolution and output 96,000fps. Hyperbolic, yes, but I honestly feel that way.
One More Multiverse filled me with hope the first time I saw it. An 8 bit VTT that was fully customizable. But no. The damn thing lags like internet explorer memes. I don't want fancy graphics for my imaginary imagination game. I want easy-to-use and easier-to-run-on-a-patoto. I can just send jpegs in chat for more details.