My guess is that the timing is entirely contingent on their existing contracts.
My guess is that HBO's contracts with cable providers have terms that are simply incompatible with a direct-to-consumer offering. If they were renegotiated at any point in the last five or so years, I would think it likely that such clauses exist. Perhaps not with every provider, but with enough to make things problematic prior to the next round of re-negotiations.
e.g.
Suppose something in the contract indicates that Comcast only kicks back to HBO a fraction of the cheapest available consumer-facing price. (Protecting Comcast from HBO's turning around and cutting a sweetheart deal with another cable operator the next day)
In which case HBO's $10 cut of the $17ish that Comcast charges [1] would itself erode should HBO make a hypothetical $12/mo offering available.
So even if HBO's cable subscriber numbers held steady, they may well be facing a huge drop in revenue simply by making an economically-feasible direct-to-consumer offering. [2]
Or, it could be a more direct clause simply prevents any direct-to-consumer offering.
But that situation sounds the most likely to me. It would have been in the cable operators interests to ask for such clauses and in HBO's interests to concede. [3]
[1] Numbers based solely on prior articles I've read. I haven't had cable, let alone HBO, in quite some time.
[2] I think we can all agree that even HBO would be dead-in-the-water among cord-cutters at anything north of $15/mo. And anything between 10 and 15 gets debatable.
[3] Even three years ago streaming was not 'ready' to the point of justifying a fight with cable operators. Even now it's anything but a sure thing. Particularly when there hasn't been a real break-out leader in the IP-set-top-box space. They'd need to sign and support a half-dozen deals and platforms to give this thing an honest shot at the moment. Going slow with HBO Go is a very shrewd way to ease into the platform fight before a crush of public interest descends on a hypothetical consumer-direct offering.