In a nutshell, because anyone either not obligated to keep something or not in a position to keep it, doesn't have the legal (halachic) status of believability.
For example if I told another Jew that I know for a fact a piece of meat has been slaughtered and processed according to the laws of kashrut (kosher), I would be believed. However if a non-Jew or even a Jew who doesn't keep kosher would say it, their word wouldn't be enough to permit a Jew to eat that meat.
(That's not to say they're lying, but in order to testify you need a certain status. Similar to having a document notarised by an official notary, or a passport picture signed by someone of specific community standing.
The criteria necessary to 'sign off' on kosher status, is (a) being obligated, ie being Jewish, and (b) actually keeping kosher.)
As a convert was once in a position where her virginity had no special status (as far as Jewish law is concerned), because non-Jews are anyway forbidden to Kohens, she doesn't have legal 'believability' that she's still a virgin.
This BTW would be the same in many instances of Jewish women being captured or kidnapped. Since they weren't in a position to ensure their virginity, they can no longer testify towards it.