The same reason that all new C projects aren’t being written in rust.

There is a lot of expertise floating around in the ~30ish years of netfilter so that is a ton of momentum in terms of the personal expertise of more experienced devs as well as the blogs/tutorials available when you search for “network filtering Linux”.

The default of kubernetes is still huge iptables chains and the only realistic ebpf alternative is mainly driven by a currently unsustainable startup (Cilium). The maturity just isn’t there yet.