Personally? I think the old SLDF fighters by and large didn't inspire the same kind of "we want to hold on to a piece of that" that some of the mechs did. Machines like the Orion and Marauder and Highlander really held a place in mechwarrior memories and they wanted to retain some of that.
By comparison it doesn't seem like they were nearly as nostalgic about the SLDF ASFs.
Perhaps it's because some of them are still flying around. They pop up all over the place in the second-line aerospace RATs of the Clans.
So maybe, especially with Royal variants, they felt like those were a good enough remembrance of that piece of SLDF legacy? And that anything new they were going to be build should actually be something totally new.
That being said, I still don't get why they didn't get a partial clan tech refit at least. SLDF weapons don't use the same ammo as Clan weapons, so you'd think it would simplify logistics at least to have all the missiles and autocannons firing the same stuff. But perhaps I'm just being too practical.
