Like most things, habit probably. Also, I used to work a lot on Safari/WebKit, so I guess sentimental value?
That being said, I do like the way Safari "feels" a lot more than Chrome. I think Chrome is actually quite ugly, and bad from a UI perspective. For example, Safari's overflow tab menu is a much nicer solution than Chrome's insistence on shrinking tabs ever smaller until you can't tell them apart at all.
Additionally Chrome is missing a killer feature I use all the time: zoom, and more specifically, double tap to zoom. I find it hard to read a lot of text on websites, and double tap, centered, zoom, is amazing. Its too bad everything else feels like its gotten way worse in the last 5 or so years.
I also use Safari pretty much exclusively because I like the UI over Chrome, but the one feature I love about Chrome is its tab closing/resizing. When a tab is closed, the remaining tabs resize and shift appropriately so that your mouse is over the next tab's close button. (See http://www.theinvisibl.com/2009/12/08/chrometabs for a nice explanation.)
I actually prefer Firefox's approach to multiple tabs: the tab bar scrolls (you can use the scroll wheel). That's about the only thing I like in Firefox anymore though… Been using Chrome because of its multi-user support (tied in with Google accounts) but Safari is soooo much better than Chrome in terms of UI and UX… Latest Chrome betas, with the full integration of the address bar as a full-blown "Google box" (e.g. you start typing, it loads Google Instant Search without the page's classic search box)…
I meant "smooth zoom" I guess? I can pinch in from anywhere on the page and center in on any particular item at any particular zoom level. Unless I've missed something, Chrome seems to have "integral" stepwise zooming, which also always zooms from the center of the page (vs where my mouse cursor is). It ends up feeling like Safari's old zoom which would just make the individual items bigger (as opposed to actually scaling the page), but I'm pretty sure Chrome is indeed scaling the page, just doing it in a way that I find frustrating and less useful.
Another example, I'm watching an embedded YouTube video which thus annoyingly doesn't allow fullscreen, so I double tap it, and it fills the browser window, and it seems to be smart enough to render at correct resolution at that scale (since it still allows you to choose a higher resolution even though it won't full screen).
To do similar feats with Chrome I have to manually jigger things around.
Safari is the only browser that has decent scrolling performance on a Retina MacBook. All the other browsers stutter really badly as soon as the page has any complexity with fixed elements (Facebook is a big one)
That's a case where Apple has to optimize the heck out of Safari until it is workable on that particular new machine, otherwise it would get terrible reviews and hurt the overall brand.
Chrome doesn't have the same incentives to get that particular machine working well, especially since the next revision of the Retina Macbook probably won't even need special handling since it will surely have a more powerful GPU. They can just wait it out, similar to what happened with the iPad 3 vs iPad 4, where people had to optimize for the iPad 3 and then those optimizations were unneeded on the iPad 4.
I can confirm what tolmasky said about WebKit2, yet I'm also a Safari user. The interface just feels better for me, and Chrome has actually been getting worse in this respect. When I stopped using it, a few versions ago, it had started feeling more and more like a Chrome OS VM than like a browser on my Mac. I doubt they've changed course since then.