CalyxOS supports more models of phone than does GrapheneOS. Not a lot more, but more.
I use it on my both my daily and development devices, I understand why some people want to use CalyxOS or LineageOS forks for the flexibility they provide.
GrapheneOS is security focused and I hope it stays that way.
Another reason is that the last time I mentioned this same reason some 'strcpy' guy apparently from grapheneos came here foaming at the mouth that I was part of a conspiracy against them. Which I'm not. I just happen to like microG a lot, in particular the local and Mozilla location database options which avoid leaking my location to Google. It's not even directly related to Calyx. But that reception cemented my opinion.
But pixel phones are hard to get in my country so I don't own one. So either option is moot. LineageOS does ok anyway. Even though they also seem to hate MicroG for some reason.