To me it seems very unnecessary to have ended beta phase 1 such a long time ago and to still not have beta phase 2 up and running. I have a few theories why they would do this.

1) Seeing if people would reactivate or start playing their WoW accounts once again. This would allow Blizzard to compare their products income potential. If for example 100,000 people reactivated their WoW accounts once the beta went down Blizzard may take note that it may be in their best interest to support games with monthly fees. I doubt they would use this information to make a monthly charge for Starcraft II as many people would not play it, but hey an extra 1.5 million dollars for shutting down beta and getting more WoW subscriptions never hurts. Maybe they will make Battle.net itself cost a monthly fee and take away the subscription charges for WoW. So no matter what Blizzard games you play you have to pay a monthly fee. That would be a pretty evil plan by Blizzard, but I wouldn't put it past them.

2) Blizzard staff took a collective summer vacation. They have been working on this game for such a long time. They figured well once the game launches were going to have to keep working. We might as well take a few weeks off and party and relax in the sun.

3) They actually have some serious issues that they are trying to figure out and fix. Most likely all those games where both players lagged out, and nobody wins and all the Battle.net bugs. Such as Battle.net saying you have to redo your placement matches even though you don't. Or perhaps making Battle.net somewhat functional and practical to use. No chat rooms, a idiotic friend system, no clans, no cross realm play. Maybe they freaked out and decided they should actually START making Battle.net NOT SUCK. I somewhat doubt this scenario because I fail to see why you must take a beta offline just so that you can work on it. This is the most professional and reasonable reason why Blizzard took it down, but for some reason I doubt this is the real reason.

Well there's my thoughts on the matter for the moment. Feel free to add your theories why Beta has been shut down.

The reason is - the infrastructure had to be prepared for the release and the huge number of players logging into our servers. A lot of minor bug fixing, perfecting small details and all the different things necessary to deliver an awesome product. Once it is all done, phase 2 will begin.