I also think there are two or three other factors that have at least a little impact:

Grid is kind of hard compared to flexbox. We might argue that it’s not bad, and the learning materials around Grid are really wonderful. However, there’s conceptual leap going from two to three dimensions, plus new syntax (and middling IE11 support like someone said), that learning and implementing is more work than any perceived benefit.
Bootstrap doesn’t support it yet. The abundance of flexbox-based grid systems make implementing grid a no brainer, and especially since the grid classes are almost one-for-one with the ol’ 960/1140-based grid systems before it, that was as uper easy transition.
Most sites don’t need it. In fact, I’d still argue that the downward pressure on site design is down to a single column. For most folks, being great on mobile is more important than being neat on a larger screen, so that’s where the energy goes.

I don’t know if I really see CSS Grid getting much more adopted than it is now until a major framework (bootstrap 5?) makes it the default of their layout system.

I don’t see much SMIL usage – there are ideas from there like [switch] in svg, things in css animations or VTT – this is really SMIL-ish; we cant use it because no browser had implemented this – that is not a great loss – everything can be reproduced by js

One reason I see for using flexbox way more than Grid is the support. If you have to support IE, grid becomes a real pain.
But when I see people using flexbox to centre a single <div />, I’m even more in pain.

For me it’s the middling IE11 support that keeps me away from using grid.

Yes, those things are true. Sounds like you think the “spreading content evenly” is something that more sites need than the different things Grid does. OK. I think the opposite. Although really I can’t imagine building most sites without both.

Mostly I think more people just know flexbox better hence the higher usage.

Oh cool! What else can you use SMIL on?

Holy buckets. Flexbox on half of sites and grid only on two percent?!

Well, isn’t that implicit? Flex is useful on most lists where grid is more a specific layout thing. To create a flexible layout that expands with its contents is way more elaborate with grid than with flexbox. Grid relies on defined boxes, whereas flexbox mostly spreads content evenly.