If you want to know why, the reason is based in who votes Dem in each county. The Inland Empire democratic party relies very heavily on Hispanics who drop off in off-year elections. While Orange Dems also depend on Hispanics, it is the Asian vote that powered the Blue wave down there in 2018. Asians didn't go as hard for Dems as they did for Clinton, but they mainly stayed blue (outside of local Asian GOP races) and mainly kept their turnout levels proportional to the rest of the population. So OC went blue whereas Riverside whites overpowered their Hispanic neighbors.
Despite the easy answer to 2018, your main question in regards to 2020 is still an interesting one. OC has more voters available to flip blue and has more people moving in with democratic leanings. Riverside is less flexible. However, Riverside has been Blue for longer then OC, so it has a head start. If dems are able to get Hispanics to vote with higher rates for some reason, that will be felt more in Riverside than in OC.
Makes sense, though I'm still curious: Why did Riverside vote to the right in 2016 in that case?
Riverside County is one of the few areas of the Southern California metroplex with a significant White Working-Class demographic.
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