More speculation. Having watched the trajectory of careers, both local and national, marred by allegations of criminality and spoken to many prosecutors, I have concluded that a significant degree as to who gets indicted is how big an SOB one is. A non-political example - Martha Stewart. Her "crime" is not always prosecuted, but she was such an SOB to staff setting them up that I think that factored in how she was treated.
Again, we don't know yet about the quality of the evidence against DeLay. But I feel certain that there the GJ testimony painted an unflattering picture about him as a person. And that matters, I strongly believe. The GJ may have asked the SA whether he could be charged resulting in the amendment to seek conspiracy charges. Delay was a classic bully that did more than exercise power to an end - he revels in pure meanness and attacks against opponents far beyond what is strictly necessary to accomplish his goal. Plus he has sold out so many.
Even in civil law, I have learned that the "quality of memory" of witnesses can vary based on how they were treated by the individual. Time and time again I have seen businesspeople that are abusive to staff suffer when those individuals are subpoenaed to testify in a case, and vice versa. When the target targets people right, the witnesses try to be honest, but tend to remember any helpful detail. When the target of testimony is a jerk, suddenly the witness will testify only as matters that are precisely remembered, and add no helpful context. And juries pick up on that. Also, when you have spent your life going out of the way to make enemies even when that wasn't necessary, don't expect anyone to stand by you when the chips are down - the daggers come out, to use the metaphor.
Methinks Delay is paying for a multitude of sins, not all of them legal