I am told there was such a time. It definitely doesn't seem that way atm. Plus, with the intro box always featuring marines of some sort it shouldn't be surprising that marines are the army of choice of many new players, and GW tailors the rules about them. Generally few, resilient models, few uses of the more obscure rules... why wouldn't they be the beginner's army, and why wouldn't GW factor that in their design?
I think that might have to do with the lot of frequent updates and attention. Space marines and the Imperium are the designated protagonist, so to speak, and there are a lot more ways to get an army started cheap and easy such as more packs, starters, etc. That said, I'm not so sure just how much money GW is getting from what army and what is keeping their company budget. Would their company bottom line be that much worse if there were half the 4-stat, 3+ armies and the remaining attention (and updates) went into orks, chaos, and nids? I'll pass on the Eldar, I might be biased there

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As for Imperials adding variety - compared to no army release at all, perhaps, but compared to other codices? Just look at how many marine codices there are - despite their differences, they tend to share a lot of things in common, right down to vehicles whose only difference is how much each author costed the unit and upgrades due to the developing balance and their own perception. I'd actually go out and state that even CSM aren't as different from other marines are marines are from the guard or nearly any xenos army. Orks and tyrannids offer options to swarm the board, Eldar of varying spikiness are fewer, but a lot more fragile than marines and require more positioning and maneuvering, Tau tend to be a specialized shooting army that is imo more like a lower numbers IG, Necrons can offer swarm or specialized mech options.