Who are "those"? Non-xian Jews? So xianity likely was regarded as the Mormons of Judaism at the time (i.e. a splinter, adding another tome to the "established" doctrine). Still not sure why they wouldn't heap more blame on the Romans.

Yes - those Jews. The Jews who made up the crowd who weren't followers of Jesus.

As for the Romans, when Matthew was written it would have been shortly after the end of the first of the Roman-Jewish wars. The Romans would have just finished laying siege to Jerusalem, crushing the Jews in a brutal repression, and destroying the Second Temple in 70 AD. About a third of all the Jews in the region were either killed or taken as slaves....but the destruction of the Temple and loss of access to Jerusalem was a massive earthquake for the Jews. It destroyed one of the fundamental pillars of Judaism - for 500 years, Jewry had focused specifically on sacrificial worship in that specific location in Jerusalem. Now that was gone, forcing a shift away from five centuries of tradition and towards the religious structure we see in contemporary practices.

In the aftermath of that kind of horror, it would have been...impolitic to lay the blame for Jesus' death on the Romans. And that's around the time that the different sects of the Jews were really going their separate ways, since the loss of the Temple eliminated a common point that had moored the different sects together (and some that were more integrally associated with Temple worship disappeared altogether).