GrapheneOS developers are free to set their bar wherever they like it. It's an independent, non-profit foundation, driven by community contributions. They provide a web-based, hands-free installer. They offer their work for free, and owe nothing to anyone.

Personally, I wish there was an open/libre device on the market that GrapheneOS could target.

Suspicion constantly comes up in this regard, but their site (as linked by another commenter) provides their rationale.

The last cellbrite leaks show it as more secure against attacks from le than the current iphones, and that's more important to me than abandoning google hardware.

Serious question: can you point out some serious complaints? They seem to have an exhaustive justification for their reasons to only support Pixels, see https://grapheneos.org/faq#future-devices

Calyx development has stopped.