The key term here is BETA. Heroes of the Storm is so new and has so few champions and skins that it really hasn't become a hassle yet. Now, I will say that it really does not seem like a large undertaking when you include lore behind skins as you're releasing them, but for them to go back and add lore info for the hundreds of skins they've already released, I hope you could see how that could be a bit of a frustration.
Don't forget that Blizzard also has over 20 years of established material to work with.
I was discussing a similar issue with my fiance (he's playing HotS next to me right now, actually) about how disappointing it is that they did away with the Journal of Justice, among other lore-related candies. I really don't think that it would be that difficult to get a group of volunteers from the community to keep up the legend. In fact, I think multiple projects have already been started. But if Riot were to feed them some information and general guidelines in-between when a champion is announced and when they are released to come up with an article, it'd satisfy those of us who are interested in more than just gameplay. Not to mention that there are thousands of people who would jump at the opportunity even with an NDA requirement.
The grand majority of us love to be a larger part of something that we thoroughly enjoy. The other 10% are playing single player games on their emulator boxes.