Out of those 15, 10 are freshmen who were newly elected in 2018 elections. 1 (Conor Lamb) is a relative newbie since he won Special Election in March 2018 in another House district. 4 are House Veteran.
Out of the 11 freshmen (including, Lamb) who did not vote for Pelosi, there is only 1 who represent a Democratic-leaning district (CO 6, D+2). The average for PVI for those 11 districts is R+3.6. It is not hard to see why they would not want to be associated with Pelosi whom many conservatives considered as far-left.
None of the 4 House veterans who did not vote for Pelosi represent a Republican leaning district. However, they are not solidly Democratic districts either. Two of them are even in PVI.