Because it discourages co-operation between modders. Not many people I imagine would want to make a framework mod that others will turn around and profit off of.
Also, all the issues with mods breaking due to patches and other mods conflicting isn't a problem of needing more money or a rating system. The problem is that you have a bunch of different developers all creating add-ons for the same game and not worrying about conflict issues with most other mods, except maybe some of the more popular ones. Trying to run 100+ mods from 80 different people is always going to be unstable at times.
Plus, a lot of us don't believe that it would result in better modding tools or documentation from Bethesda, and Valve doesn't even do anything beyond providing a place to host the files. More money to Valve wouldn't provide us with anything of value.
That was my biggest concern as well.
That modders should have the option to be paid for their work if they want to is fine if you ask me. Demanding that people work their ass off for hundred, even thousands, of hours so that you can get free stuff is silly.
However, if Valve and Bethesda are putting up mods for sale, I definitely expect some sort of upside to it. A carrot to go with the stick, as it were. And there was none. Not only do you get no help in case of compatibility issues (which are legion are soon as you start installing big mods), but the Steam Workshop is an absurdly crappy platform to get a clean install for mods. It's slapdash in almost every possible way.
If mods are free, having to figure that stuff out yourself is fine. But if you put a price on them, the game changes. You need to to at least provide some amount of documentation, QA control, and customer service, for customers and modders both. And none of those things existed. There was no upside for the customers in this deal, save for a vague and highly questionable promise that it would increase mod quality somewhere down the line (yeah, right, like that worked for Greenlight/Early Access).
Coupled with Valve's lack of communication, this idea was basically implemented in the worst possible way. It does not surprise me at all that it failed so fast in hindsight