Presumably because there are a significant number of Republicans who want to see Feinstein gone but would obviously prefer to vote for a Republican. When given the option in polling to choose R, they're going to be more likely to choose that.
Anyway, it looks like most of the non-SUSA polls didn't include any Republicans. Tthe Berkeley one did, and you can see that KDL's numbers dropped from an average of 19% in the previous polls to 11% when one GOP option was included. With two GOP options, he was at 8% in SUSA's.
However, I think Feinstein
could be in serious danger if it ends up being D vs D, for many of the same reasons that Bernie was able to attract so many votes in states like WV, OK & NC during their closed primaries. Granted, I'm sure there'll be two primary camps in the GOP in that scenario: one will be anti-Feinstein above all else and vote for KDL, and the other will see Feinstein as the lesser of two ideological evils. In the end, it might not make a difference, but I could easily see as many self-identifying Republicans (%) voting for KDL in a D vs D matchup as Democrats.