If bandwidth/capacity is an issue (which it was until D11 came on line), they have to pick and choose, which is done based on viewer preferences (on the large scale), combined with negotiations with the content providers. If the cost of channel x is three times the cost of channel y, and roughly an equal amount of customers would watch either/each, then channel y will be provided.
Now that bandwidth/capacity is not a constraint, it all comes down to negotiations with the content providers, as well as what content is actually available in HD. If a channel provides HD, but only has 10% of it's programming in HD, it might get a lower priority for being added - take longer before you see it. I would expect that DirecTV will offer virtually all possible HD content as soon as they are able to do so (have completed negotiations, and the content is available).
Carl