Yes, Feinstein likes a big-spending government, and prefers more of that spending to go to social programs than Bush did, but that doesn't make her a "leftist" any more than it made him one. She's pretty consistent in her opposition to most of the Bill of Rights - doesn't like free speech on the Internet, doesn't like search warrants, didn't stop torture at Gitmo. Yeah, she's occasionally come out to support the liberal side of issues like gay marriage or abortion, but it's not like she really broke through a lot of boundaries on those in the Senate.
Basically, the only thing leftist about her is that she counts as +1 in the "D" column instead of the "R" column when you're counting the Senate majority party, which does make a huge difference. But she's got enough seniority that California's Democrats haven't run a serious primary challenger against her in years, and California's Republicans know they're going to lose so they've only run token candidates who are doing their "take one for the team" job in return for future party favors. It's too bad Carly Fiorina decided to run against Barbara Boxer instead of DiFi - I dislike her immensely, but it would have been a fun race to watch.