Why did druidesses vow perpetual virginity?

There is, in fact, no evidence of druidesses existing.

Admittedly the evidence of any kind about druids is meager, and what we have is often bad -- for instance, Julius Caesar tells us they have high position in Gaul, but then in his discussion of his conquest make no mention of any druid, which seems implausible if they were that prominent -- but it does not include female members.

This very lack of evidence has led to a lot of very ill-founded speculation. To convert Veleda -- a German seeress, and though they recorded she was unmarried, they did not record that this was connected with her position -- into a kind of order of Vestal Virgins would be a typical exaggeration, and less than some.

You may find Ronald Hutton's The Druids of interest.

Perhaps an answer can be found from a different group of Celtic people, the Welsh: Math fab Mathonwy was a king of Gwynedd who needed to rest his feet in the lap of a virgin unless he was at war, or he would die. (Wikipedia).

In the book Celtic goddesses by Miranda J. Aldhouse-Green, pg. 70, she writes:

The sexual imagery of the goddess can also be less obvious: in the story of Da Derga's Hostel (see p.73), the doomed King Conaire refused entry to a crone, who was the Badbh in her guise as sovereignty goddess, because he was under a geis or taboo whereby he could not receive the company of a single woman after sunset. So, even if the woman was an ancient hag, there appears to have been the peril of contamination of male by female, a sexual danger which may - like the myth of Samson and Delilah - be associated with male weakness brought about by women. The converse of this may in fact be present in the Welsh Tale of Math (see Chapter 3). The concetrated, undissipated sexual energy of the virgin may be another manifestation of female sovereignty.

And indeed, it seems that the Germanic seeresses were held in high regard:

Veleda was an unmarried woman who enjoyed wide influence over the tribe of the Bructeri. The Germans traditionally regard many of the female sex as prophetic, and indeed, by an excess of superstition, as divine. This was a case in point. Veleda's prestige stood high, for she had foretold the German successes and the extermination of the legions. (Tacitus, Histories, 4.61)

With these proposals they first calmed the Tencteri and then sent a delegation to Civilis and Veleda with gifts which obtained from them everything that the people of Cologne desired; yet the embassy was not allowed to approach Veleda herself and address her directly: they were kept from seeing her to inspire them with more respect. She herself lived in a high tower; one of her relatives, chosen for the purpose, carried to her the questions and brought back her answers, as if he were the messenger of a god. (Tacitus, Histories, 4.65)