It is true that UCLA and WUStL are at extreme ends of the list of the USNWR top 25 sorted by percentage of students on Pell grants (UCLA 38%, WUStL 8%). However, most others in the list are closer to WUStL than UCLA in this respect. Columbia and USC are the private ones with the highest percentage of students on Pell grants (31% and 24% respectively).
Of course, admission criteria and processes can affect which of the highly academically qualified applicants are admitted and can enroll. Non-consideration of legacy, not using CSS Profile, taking many transfer students, and being impressed by essays about overcoming adversity are likely to tip the admit and matriculant pool toward including more students from lower SES families, compared to typical highly selective college admission criteria and processes.