They did, but it didn't really matter to them. Do you see the difference? In Catholicism a whole doctrine is built around Mary and her virginity, while the protestants (including the reformers) think that Jesus is the important one, not Mary. Mary didn't play a major role anymore and the reformers didn't deem it necessary to mention their belief in her (perpetual) virginity. The only important thing for the reformers was that Mary was a virgin when receiving
Jesus, everything afterwards and whether until she died or not was irrelevant.
You can belief something without making it the center of your beliefs, that's why the reformers, even though believing in perpetual virginity, didn't make it part of their writings.
EDIT: That Mary was a virgin when receiving Jesus is still an important part of what we believe by the way. Just the perpetual part got lost - either because it is not necessary or because of the verse I quoted above, or even for both reasons.