I have been looking at that market dynamic for a while actually, and while I am not discounting your point, there are many reasons why 1/2 are in the top ascendant spot and also they are not necessarily or easily explained by a view that well it's because NodeBB is not as good as these.
You can not discount the existence for a long time of a Digitalocean one-click, very low ethical jump to get a discourse up and running, so the numbers, might be a bit bubbly, how they translate into deep communities, versus a lighter support like presence and interaction, is another story completely.
I am not really to up on docker but I really do not like that extra layer. IT feels like you can barely touch your installation. If you are stuck on a older version, you are also going ot hit real problems with upgrades. So if you are not too technical, you may get hamstrung here very badly.
Upgrading NodeBB is a very lite experience smoother experience IMHO.
The creator of Discourse has a much higher profile due to I guess Stackoverflow, and I'd say a lot of money and marketing has gone in there too or has been available along the way, also I think was at least a year or more ahead in terms of creation and development - not sure if importing existing forums is any easier on discourse or any other new forum platform, but that might be a factor to.
You could ask the same question about xenoforo and it is nothing like Discourse, it is more like NodeBB but really closer to phpBB/VBulletin type platforms, it is very popular but from using a few forums on this I am not too impressed, but it seems to know it's market really well and what it wants and that means a lot in this space.
Nodebb is similar to a classic forum, and the discourse is similar to a support site.
I think that is a very fair and efficient composition of both platforms.
There are problems with the nesting of categories in the discourse. The developers decided...
For me this is the actual issue with Discourse "the developers decided...", there is a bit of that's they way we like it / do it round these parts arrogance baked into the goods - infinite scroll is emblematic of that type decision that is so divisive, you have to realise that thinking is carried forward through all future development - from what I know, it looks like they will never cater to the other side on that issue and I do not know why if it is ego, or they put everything on black and it is technically to difficult to now add a no-infinite scroll option and the subject remains in the "lets not talk about the war" category.
You'll find a few more "decision" easter eggs, that will have you scratching your head. It's not a terrible platform, it's a very good platform, but sometimes, tastes and styles are not always compatible, find what you can live with.
Discourse does seem to have built up a lot of steam in recent years but I still view it as a better fit for a new community, but community is the wrong word, it's missing a lot of the little touches that engender community.
Plus you get to have interesting threads like this here on NodeBB community, I never noticed such stuff on Discourse meta forum, maybe because I'm always trying to fix something technical.