I wonder why this is, if Rome especially was kinder to women than other societies. I can see why for slaves, since it would have religious doctrine that was equalizing for them. But women have a downgraded status in the ecclesiastical structure of the early church. There were many pagan priestesses and the Vestal Virgins as you say but obviously there would have been meltdowns if there were woman priests or bishops in Christianity.
What you should remember, is that Rome didn't force her views onto her subject people. All that Rome cared about was that you kept order and paid your taxes. While Roman Law applied to Roman Citizens or Latin-Rights communities, the original laws and practices were still followed elsewhere. Hence the Jews could stone someone for blasphemy, Greeks kept their women sequestered, etc. Obviously Rome would intervene if there was a conflict between the native and Roman Law, such as when Paul called upon the Emperor and was promptly taken first to the governor and then to Rome. Now and then Rome would end native practices, like Gaulish human sacrifice, or order things opposed to them, such as when Hadrian tried to acculturate the Jews into Hellenists, by rebuilding Jerusalem as Aelia Capitolana (the latter in a mistaken attempt to pacify and hopefully break Jewish exceptionalism, but precipitating another Jewish revolt).
So the spread of Christianity amongst women in the Greek communities or Semitic ones, is not really too different from amongst slaves - They were very much subservient to their men. Romans themselves treated their women better than most contemporaneous peoples, but this does not mean the sexes were seen as equals. Women were still seen as less than men, in a way, although Rome had a strong tradition of celebrating their Matrons, who was sort of a stock figure of true Romanitas or Romaness. I'd say 'different roles' perhaps, but in our modern world even this euphemistic expression of the concept is seen as offensive.
I've seen it argued that Christianity allows a new identity, outside of the role as mother or wife, for even religious roles were bound thereby traditionally (Vestals dressed in a mix of Roman bridal attire and those of a staunch Matron). It allowed all people to be seen as merely souls before God. In a sense, one could argue that Spiritual achievement somehow allowed exercise of leadership, beyond their traditional role, as in the Montanists. We see Christianity allow direct access to God, without all the other accoutrements that usually went along with this, like sacrifices and such, regardless of gender. It was also a radical religious departure, as Roman religion was often framed in a contractual sense, of if I do this, the gods will do that.
The other mystery religions weren't very welcoming to women either, for Mithraism completely excluded them, and groups like the Manichees or Jupiter Dolicenus tended to stress their lesser status far more than Christianity originally did. We see prominent women in the NT, Pliny writes of two deaconesses that he tried in Bithynia, and as late as the 5th century there were still local synods that sent out decrees that women should not be deacons or handle the Eucharist in Gaul and Hispania - meaning that there were communities where they did so.
So while Christianity came to be less 'women-friendly' from aspects of hierarchy later, this was most definitely not the case initially in much of the Gentile world (not too sure of the Jewish Christians, who I assume imported their traditional clear subservience). It is probably related to Christianity becoming the state religion of Rome, but again this is just a supposition. After all, Helena, Constantine's mother, played a leading role initially in the conversion process according to Church accounts, and definitely was an influential figure in her son's reign.
