> why is prosecution going with that lab/software only?

Presumably becquse that lab/software gave them the answer they were looking for.

Why pay for even more lab/software testing when you've already got what you want (especially when it's a possibility that the additional testing will contradict the first)?

Even if you had plenty of DNA material, going with multiple labs comes with a risk that one of them comes with a "no match" result - and that is something that is supposed to be disclosed to defense as potential exculpatory evidence - and is a nice source of reasonable doubt for a jury. It's something that is terribly inconvenient for the prosecution when someone has already been charged with the crime...