Because it comes with all of the drawbacks of IPv6 but also ditches some of the advantages.
You still need to update every router and application. Network admins still need to learn something new. The two protocols still don't interoperate. If you're going to go through all of that trouble why only do a half measure. IPv6 is supposed to be the final version of IP.
On the contrary, they would, the behavior's and quicks would be the same. And if we define, say, that if the last four components are zero, then the addr is the same as normal IPv4 address, then you could deploy the whole thing without having anybody assigning new addresses. NAT's/configs/etc could keep working.
The big problem with IPv6 is that everything has to be double-configured to support both IPv4 and IPv6. Two addressed for all. Different semantics. No backwards compatibility.
If you imagine that all network HW is recycled, say every decade, you could roll the thing in without having anybody to reconfigure everything. Eventually coverage would be complete. This cant happen with IPv6, because the double configuration problem. Extending vs replacement.
This is of course a pointless though experiment, because IPv6 is the route that was chosen.
The devil is in the details. All applications that use the Socket interface (which is almost everything that talks on the network) still needs to be rewritten. Firewall rules still need to support longer addresses, even if you do keep the old ones--it is basically the same situation we are in now, only the line between the networks is fuzzy and there is more confusion. You still end up with two sets of configurations for everything.
> If you're going to go through all of that trouble why only do a half measure.
Because a "half measure" would have been easier to adopt, therefore would have been more likely. The strategic error IPv6 made was, I think, taking the point of view that as long as a breaking change is necessary, then increasing the scope of that change doesn't bring greater cost.
But it does, quite a lot of it, and that greater cost is the primary reason why IPv6 adoption has suffered.
You still need to update every router and application. Network admins still need to learn something new. The two protocols still don't interoperate. If you're going to go through all of that trouble why only do a half measure. IPv6 is supposed to be the final version of IP.