I don't remember the specifics, but diesel engines under a certain HP rating are exempt from the emissions requirements. Maybe someone can quote that number?
I believe the Tier 4 exemption limit is 25HP. That's likely the reason the 1100cc RTVs are pegged at 24.8HP. Anything more requires complex and expensive after-treatment systems such as DPF, DOC, SCR, DEF.
I think Kubota will keep the diesels in the RTV line but you just won't get higher HP versions unless you choose a gasser.