The phrase is missing from some important Greek manuscripts, including the Codex Vaticanus and the Codex Sinaiticus.
According to its introduction, the NRSV-CE is based on
the most recent edition of the Greek New Testament, prepared by an interconfessional and international committee and published by the United Bible Societies (1966; 3rd ed. corrected, 1983; information concerning changes to be introduced into the critical apparatus of the forthcoming 4th edition was available to the Committee).
The UBS3 text is equivalent to the Nestle-Aland 26th edition "Critical Text". This Greek text reads, for Luke 18:24:
Ἰδὼν δὲ αὐτὸν ὁ Ἰησοῦς [περίλυπον γενόμενον] εἶπεν, Πῶς δυσκόλως οἱ τὰ χρήματα ἔχοντες εἰς τὴν βασιλείαν τοῦ θεοῦ εἰσπορεύονται·
As you can see, the phrase corresponding to became sorrowful - περίλυπον γενόμενον - is enclosed in square brackets. Metzger's Textual Commentary explains here:
On the one hand, the excellent attestation for the shorter text (א B L f 1 1241 al) and the variety of positions of περίλυπον γενόμενον suggest that the words were introduced by copyists, perhaps from ver. 23 (περίλυπος ἐγενήθη). On the other hand, since Luke’s penchant of repeating a word or phrase in adjacent passages may have operated here, a majority of the Committee did not feel at liberty to omit the phrase entirely, but enclosed it within square brackets.
It would appear that the NRSV-CE translators and editors opted to exclude the bracketed Greek in this case.
This is not necessarily an innovation. The phrase was also excluded from the NRSV, RSV, and RSV-CE, which are based on the same Greek text:
Jesus looked at him and said, “How hard it is for those who have wealth to enter the kingdom of God!
Jesus looking at him said, “How hard it is for those who have riches to enter the kingdom of God!
The Modern English Version is based, as is the King James Version, on the 16th century Textus Receptus Greek text. This text doesn't have any kind of annotation (e.g. square brackets) to indicate the certainty of the text. It also has a slightly different word order at the end of the verse, but this is more or less irrelevant to the English translation:
ἰδὼν δὲ αὐτὸν ὁ Ἰησοῦς περίλυπον γενόμενον εἶπε, Πῶς δυσκόλως οἱ τὰ χρήματα ἔχοντες εἰσελεύσονται εἰς τὴν βασιλείαν τοῦ Θεοῦ.