Why do the Astros have to go to the American League?

Several reasons:

1. Create an inter-divisional rival between Texas and Houston (which means money).

1a. The AL West formerly consisted of three Pacific time zone teams and one Central time zone team (Central is two hours ahead of Pacific). Adding another Central time zone team really helps Texas' schedule and stuff.

2. Moving a team from the NL Central to the AL West is 'the' easiest way to have six five-team divisions; no one else has to change divisions!

3. The Astros were being sold, so MLB actually had leverage over someone (the guy buying the Astros), and they used said leverage to "make" him agree to the move.

4. Yes, Milwaukee would make sense (although some divisional realignment would be necessary), but Bud Selig is still in charge, and he 'really' wants Milwaukee to be a "National" League town (because of the famous Milwaukee Braves). So...

The reason why the Astros are going to the AL is part of the latest collective bargaining agreement between MLB and the players as to make both the AL and the NL with equal amount of teams. Each league is going to have 15 teams and each division is going to have 5 teams apiece.

The Astros were chosen because the AL West only had 4 teams in the division and the NL Central had six. Since the Astros were the geographically located team in the NL Central, that is the reason why they were chosen.

Personally, I would have moved the Rockies or D-Backs into the AL, rather than the Astros. Milwaukee was an NL city for years, so the Brewers being in the NL makes some sense.

But, the Astros were the team that was sold, so the Astros are the ones who were moved.

The main reason was to balance out both leagues with 15 teams and to promote more rivalry between teams. (Rangers, Astros) Being a Astros fan I hate the move but it will be good for Houston to see the Yankees, Red Sox, and Angels. It helps smaller teams to expand there market. I do agree i don't like two Texas teams in the A.L. either

Bud Selig wanted to move an NL Central team to to the AL West to even out all the divisions. So both leagues have 15 teams and all divisions have 5 teams.

mainy to even out the divisions

and possibly for expansion in the future when two teams are added and then they are divided into 4 divisions for each league

I basically agree with you. It boggles the mind. Why bother moving Milwaukee in the first place then?