@jpitts: adding some background from the EIP here.
This is motivated by Eth1.x / Stateless Ethereum discussions, more specifically discussion around the “witness format”.
Introducing a restriction would allow storing the nonce in a more optimised way.
Additionally it could prove beneficial to transaction formats, where some improvements are potentially sought by at least three other proposals.
Lastly this facilitates a minor optimisation in clients, because the nonce no longer needs to be kept as a 256-bit number.