Because it helps keep us straight for phases when Corsas could be either Vauxhall or Opel - it's easier to have all Corsas of the same underlying generation with the same numbering, rather than out of step. Putting Mk1 on Novas is useful as a marker that something's going on, and hopefully deters folk from inventing Nova Mk2s.

The other issue when pondering Carltons, Rekords, Omegas etc is that we have a system which everyone is used to. If we change established practice (a) it'll usually generate hiccups and (b) it should be to bring actual improvement. "Different" does not necessarily mean "better" in these scenarios.