My guess would be that it's all done in pursuit of the fabled "balance".
You take two ships that are, ostensibly, similar and you'll realise that a ship with mostly odd-sized slots is actually going to be useful for different things than a ship with mostly even-sized slots.
By way of example, it's why the T9 can be a more attractive mining ship than the T10, even though the T10 is superior in a bunch of ways.
I guess it'd be nice if FDev did just create a heap of modules in all sizes but I can see why they haven't and it's kind of fun (for me, at least) to plan a build that successfully overcomes all the compromises and limitations of a given ship.
Sometimes it's not good to give people everything they might want. It creates an opportunity to reward people for thoughtfulness and ingenuity.
Course, it wouldn't hurt if some ships were deliberately a bit more biased toward the roles that they're apparently intended for - if, for example, the AspX and DBX (the clue's in the name) had more slots that were useful for exploration.
As I've said before, I think it might be a good idea if, as well as the default slot-capacities for ships, FDev made slightly modified versions available as a reward for achieving various things related to, say, exploration, combat, trade and mining.