The question is: why didn't the prosecution test it, and when it was discovered by the defense why did they (and Maresca) protest it being tested throughout the rest of the trial(s)?
The answer is (most likely) that they
did test it. After all it was a
visible stain found on a pillowcase underneath the hips of a mostly nude murder victim who had the DNA of an attacker insider her. One cannot pretend they didn't look closely at that pillowcase, after all they tried to advance one of Rudy's partial prints as evidence of a 'size 37 woman's shoe' (or however they put it) thus they must have examined it thoroughly.
However like the TMB (a blood test) negatives on the luminol hits but omitted from the (supposedly) comprehensive forensic report, the RTIGF, they probably just decided to omit the results of the test on the putative semen stand as well because it 'didn't help their case.' Regarding those TMB negatives we even know they (Stefanoni) lied in court about doing them and would not have been caught had they not been ordered to release a bunch of documents on July 30, 2009 when the trial was mostly over (all of August is a vacation for the Italian Court System).
There's
over a hundred samples that returned results indicating about as much or more DNA than (what they claimed was on) the knife blade, but those results were also withheld and the prosecution and Stefanoni fought tooth and nail to ensure the court or public never saw them.
That semen stain would have been tested by the Massei court had the prosecution and/or Maresca insisted, but they took the opposite tack and fought against it instead. Why do you suppose that was? Why did the prosecution and Maresca jump up and protest when the independent experts asked if they could open up the kitchen knife and test areas in the cracks where blood couldn't have been washed up? Normally you'd expect the
defense to protest the possibility of new evidence being developed during the trial, but in these instances the defense either asked for it to be tested (semen stain) or didn't protest (opening kitchen knife).
The most parsimonious answer is because the prosecution already knew what those results would be and that they 'wouldn't help their case.'