As some have said, Frontier claim an offline mode is "too hard". That's not really the truth.
It's much the same as the fuss about Ubisoft not having having a female lead in AC::Unity and the complaints we saw about that from nutters (the complaints about the game not being any good are a whole different matter) and Ubisoft claiming it would take too long. It wouldn't, not just for mo-cap.
It almost certainly would take too long to add a rewritten version of the whole story so that it works with a female protagonist and Ubisoft have (EA-led) deadlines and probably penalty clauses for failing to deliver on schedule. That doesn't matter to the professional complainers, of course, but it should matter to gamers who just want to play the damn game.
By the same token, Braben & Co have to deliver Elite to a timeframe and they've dropped a single-player campaign to do so. Why couldn't they release the single-player campaign later? Well, it's a fairly integral part of an initial release. People play single-player before hitting multiplayer. Adding it in later would be weird and contradictory and almost certainly raise just as many complaints as dropping it entirely. Kickstarter backers - let us not forget - are not known for their quiet, understanding patience. They are internet fucktards like all the other internet fucktards, so quite the reverse.
I think Frontier were in an impossible situation. They could go way over timeframe (eliciting calls for refunds everywhere from everyone), they could patch in offline mode later (eliciting exactly what we're seeing here but with added development cost and no added funding) or they could pull offline mode completely which still causes internet fucktards to throw all their toys out of the pram but at least it doesn't actually cost them yet more money and time.
On balance, they made the sensible choice. Backers don't like it? Meh. Kickstarter is not buying from a shop. Backers should be thinking themselves lucky Frontier delivered at all.