When you have proven you can't implement diversity in the lineup you already have, why would you want to add more problems to an existing one?
By making every ship too overlapping with others in ability and purpose, they've devalued the "diversity" of their lineup and poisoned any hope for truly niche ships in the future. As weak minded players will just demand the same abilities they've had in all other ships to be available with the new ones or they'll just never be used.
What a load of crap. That's an FDev problem, not a player problem. Its their responsiblity to come up with interesting new ships, not the players. Or they should make some ships completely customizable so they can do any job the player wants. But for players to be hostile to the very idea of new ships that may or may not overlap other ships is just stupid.