because Asik is better than Anderson
Not really, although he's a better fit. While Dwight has shown success with a stretch 4 (though Lewis was better than Andersen), Houston needs rebounding and defense 100x more than more offense.
We're not talking about teammates like Nelson, Lee, and Hedo who need a Dwight-centric offense to get them open shots, we're talking about an MVP candidate in Harden. The ball should be in his hands at all times, he should be making the decisions, not Dwight. Use Dwight as the elite pick and roll player he is, don't force him to post up and make passes. The receiver/finisher, not the initiator.
Houston has probably the first or second best chance at making it out of the West, let them prove they actually need help before you go changing their whole look. Who do you want guarding the paint against LeBron and Wade, Dwight and Asik or Dwight and Andersen?
Asik is better than Anderson.
Again, it's a fit thing, Asik has better strengths but only if the team is able to cover for his weaknesses. Andersen commands a defender in his skin at all times within 24 feet spacing the floor like only a couple other bigs can do, whereas Asik can be completely ignore outside of 5 feet. You also can't play him late in the game because he's one of the worst foul shooters in the league compared to Anderson who is one of the best.
If Asik continues to play like he played last year he's probably has the edge. I'd like to give him more than one year to see if he can sustain 10 and 12 on 54% shooting though.
In some ways they're mirrors of each other. Asik has stone hands, zero passing ability, and zero shooting ability. Basically he is a lot like Tyson Chandler offensively.
Ryan Anderson is not a good defender, and is prone to getting bullied by bigger 4's, and has trouble rebounding defensively for his position.
But Asik's defense and rebounding were among the best in the entire league last year. Him being on the court for the Rockets was the difference between a top-10 defense and a bottom 3 defense.
Anderson's shooting is definitely a great thing from the 4, and his nifty midrange game is awesome as well, and his offensive rebounding is pretty good, but overall his contributions without a defender of Dwight's caliber down low to compensate are a step down from Asik's.
Asik's just a more valuable player, and its not because of fit, its because he impacts the game more consistently and more positively than Anderson does. Asik supercharges your defense, while Anderson supercharges your offense.