I’ll take a stab at answering this question, since I’m a CA parent.
Over the past few decades, simply the lack of prestige. It’s considered the 8th best UC (out of 9). And also Riverside, CA is located in the Inland Empire, so it’s hours from the coast and Riverside County hasn’t always had the best reputation. Just not as nice and/or quirky as Santa Cruz, Westwood, Berkeley, San Diego. But more recently, UCR’s reputation/prestige (and the City of Riverside) has really made huge strides. And I’ve certainly changed my opinion.
I think UCR is a great option, if the OP can afford it.
BTW, USNWR ranks UCR #91 and Chapman #125.