Mostly the fact that GrapheneOS only works on Google Pixel hardware currently and vendor unlock status. It's the only available phone hardware that provides full bootloader unlock capabilities AND suitable security protections baked into the secure enclave and boot process, including things like rate limiting in hardware like password cracking attempts via external brute-force input means, lockdown of usb ports until boot unlocked with a pin, etc. Their website spells out all the reasons.

Other phone makers could if they wanted to do the same, but do not as an active choice, or at least somebody's choice above them.