The same thing that's going wrong with IPv6 adoption.

Edit: It's true. RPKI works, it's just harder to use than not using it, and network engineers have been doing bgp without rpki since forever. Managing PKI is outside the scope of a Cisco flat file and so it's not getting done. Route maps are easier to write than managing PKI. In the same vein IPv4 networks work and have been done since forever and as long as it's easier to shim over your v4 core and do carrier NAT and memorize dotted quads v6 will not get done with haste.