Or HaChaim on Numbers 9:6/אור החיים על במדבר ט׳:ו׳

1And there were (literally, was) men, etc.: One needs to know why it used the singular form for the many. And also why was it necessary to say, and they could not make the Pesach offering, since it informed us that they were impure. And maybe it is from the angle of that which God commanded on Rosh Chodesh (the first day of the month) about the Pesach offering, as they explained in Pesachim 6a, that the statement (Numbers 9:2), "And the Jews shall make the Pesach offering," was on Rosh Chodesh Nissan. And it would have been proper to guard against impurity [to reach the day of the offering, the fourteenth of Nissan, in a state of purity]. And one might attribute a defect to them that they did not keep the commandment of God and that they became impure and did not concern themselves with the commandment of the Pesach. Hence the verse teaches to say, And there was in the singular form, to say that there was only one matter of impurity by which [all of these] people became impure. And that matter of impurity was not in their power to guard from and they became impure against their will, as per their statement, may their memory be blessed (Sukkah 25a-b). Whether according to Rabbi Yose the Galilean who said that they were the carriers of Yosef's coffin, whether according to Rabbi Akiva or whether according to Rabbi Yitzchak who said they were impure from involvement with a corpse that required burial (met mitsvah), they needed to become impure - even if they knew that their impurity would last through the fourteenth. And this is to what the statement, and they could not make the Pesach offering hints to, saying that they could not guard themselves from impurity to make the Pesach offering. And about its stating, on that day, the rabbis, may their memory be blessed, already made a precise inference (Pesachim 90b) - for one master according to his opinion and for [the other] master according to his opinion. It also wanted to say by using the singular form that it was from the angle of their being individuals that that the could not do the Pesach offering, but if they had been the community, they could have done the Pesach offering, as it can come in impurity [in such circumstances]. Later I saw in the Sifrei of the rabbis, may their memory be blessed, that they made such a precise inference.