I think what's harder is to figure out a commercial model where you can manage to pay for the infrastructure and resource usage and write the associated client software.
The client software is the big one. ModDB has the others but is all but dead save for a few old titles. Vortex really did Nexus a lot of favours; it's turned into a great program, making modding easy for non-techie users.
I don't want to use their bloatware launcher. Games already have enough bloatware launcers as it is, I am not willfully adding another one. Fortunately I can still directly download from the site, but I have to log into an account and I hate having like a hundred accounts.
Also, I just don't like the idea of one website having a near monopoly on mod hosting. I would prefer multiple places, ideally ones without requiring an account or pushing bloatware.
It doesn't though? There many games for which I use ModDB and many games have modding communities on dedicated websites.
Some of the biggest modding communities (GTA, Minecraft) don't really use it at all. It's very popular with TES and Fallout (not surprising considering the original name was TESNexus), but as someone who has spent a very large amount of time modding Bethesda RPGs, many good mods aren't found on Nexus, even for those games.
I love Nexus. I uploaded 3 mods over the years, and with their donation point system (you get points each month based on unique downloads), I got like 15 free games from their store by this point.
At least you can get the mods from Nexus. If you have the GoG version of a game and the mod you want is on the Steam Workshop, that royally sucks.
(yes I know you can get most of them with SteamCMD, it still sucks)
Considering how much time and effort NexusMods has put into creating a very good website to host the mods, AND a very good mod manager (not the first iteration, that was really bad) to download and install the mods, I have nothing but respect for them. Granted, I bought a lifetime from them, because I had been using the site since I was a child learning what modding even was. Now, when I tell me friends about how easy it is to grab a Collection, and it installs everything the way it needs to be so you can just get to gaming, I love them even more. Getting people to try modding their games was frustrating as hell before because not everyone spent their entire adolescent free time on the computer trying to install mods that were never going to be playable because you were too young to understand why your computer was not powerful enough to run it!