I would love to hear from someone with the real answer but I've always assumed it is legacy baggage from the old days. The impact on performance has probably never been big enough to compel the entire x86 ecosystem to make them coherent.
I imagine it's because the hardware doesn't have enough context to know which virtual address spaces exist in which cores' TLBs. ASIDs were only added in the virtualization instructions on x86, and I can't think of an OS where those are the same namespace across cores.
A side reason is that it'd probably heavily complicate the top level TLBs to have "please flush yourself" coming from anywhere other than the core they're attached to, and those are in the critical path between L1 and L2.
Totally spitballing here FWIW, I haven't been part of the design process for a TLB.
