The question isn't why the President wouldn't invade with 10,000 troops in September. The question is why their weren't more than 10,000 troops in September, and why it has taken so long to build to the force levels now present, especially considering the weather-imposed attack window. Are you suggesting that the Administration didn't see the Turkish intransigence coming, and didn't plan for it? Or that they just didn't think about the implications of the coming summer? No, this was timed to come to a head in the UN - with full realization that the UN wouldn't endorse war - late enough in the year to appear to "force us" to postpone a major assault.
Maybe I'll have a lot of egg on my face in a month, but I honestly don't think we'll see troops in Baghdad for at least 6 months. Maybe some incremental steps - occupying the oil fields, supporting an uprising in the northern Kurdish territory. But I don't think we'll see a blitzkrieg to Baghdad. We can't hit Saddam yet, we need to give him at least the impression that he still can get what he wants - to stay in power - until we are able to neutralize the possibility of a WMD retaliation against Western civilians.
The question isn't why the President wouldn't invade with 10,000 troops in September. The question is why their weren't more than 10,000 troops in September, Given the searing weather in the Gulf during the summer months, I think it was better to stage troops during the winter months. Bush is cutting it a bit close to late April for my liking, but I don't think it takes an ulterior motive or conspiracy to explain the current timeframes.