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.
The Berkeley poll actually included all of the candidates that are running. It mentions this in it's methodology, but it doesn't show the results for all of them. If it is D vs D, Feinstein will easily win. Once Republicans learn anything about KDL, they will be voting for Feinstein. Also as much as some Bernie Democrats don't want to admit. Feinstein is popular among moderate Democrats and Independent voters. Whether it's D vs D or D vs R, I don't see her winning by any less than 60%-40%.
