Still as long as Android 15 still gets security this doesn't matter too much.

It does matter, because Android Security Bulletins only contain fixes for high/critical vulnerabilities. But all the other vulnerabilities can be useful in exploit chains. Add to that that ASBs have a three month embargo, but GrapheneOS and Samsung roll all/some patches out before they are in a security bulletin. So phones like the Fairphone have critical/high CVEs have been known for up to three months for anyone that looks.

but the main reason is likely simpler:

I think the main reason is that they do not do most hardware and software development by themselves, it's done by their Chinese ODM T2Mobile, for which Fairphone is probably just another customer.

Everything is at glacial speed. For instance, Android 16 on FP6 has some IPv6 bugs that breaks WiFi connections after a few minutes for a substantial number of their customers [1]. Six months later, they still haven't been able to properly fix it.

[1] The issues itself is probably not restricted to WiFi, it's that some brands of WiFi routers trigger one or more of the condition. One of which is sending a router advertisement with a lifetime of 0 for the IPv6 prefix used by the network. The connection handling code goes in a state where it misses the next prefix advertisement.