I am not trying to diminish anyone at all, but I would think these kinds of party line attachments mean that not everyone ends-up with the most capable developers, particularly so if what drives this is passion rather than a skilled software shop that with proven capabilities and experience across a range of relevant domains with an equally proven track record of delivering quality, well tested bug-free product.
Of course, the parties would have to we willing to pay more than hobby money for what they need.
This is a real phenomenon, but not in the way you think. Democrats have historically had a tech edge, because the real talent leans left politically. Obama kind of killed that, but for a good cause - he diverted a lot of those politically-driven types into working directly for the federal government under the umbrella of USDS and 18F.
As far as why people don't sell to everyone, the above explains a lot. Another explanation is that political sales (which is different from true election tech, which is purchased by governments) are driven by trust and personal relationships. Once you work for one party in the US, the other will not trust you.