I hear many Rivian customers really love Comma.ai, so much that they are #1 on Comma dash.

Probably a lot of overlap in the venn diagram of people who would like the two things. Mostly the "Early Adopter" circle.

Also a lot of cars have a lot of limitations with comma.ai. Yes, you can install it on all sorts but there are limitations like: above 32mph, cannot resume from stop, cannot take tight corners, cannot do stop light detection, requires additional car upgrades/features, only known to support model year 2021. Etc.

Rivian supports everything, it has a customer base who LOVE technology, are willing to try new things, and ... have disposable income for a $1k extra gadget.