Earlier quoted context omitted.
It might have been better at the time to use gecko, but apparently building a new rendering engine had enough advantages that Apple decided it was worth it. In hindsight, WebKit was the right call. It's powerful enough to run advanced desktop browsers, yet lightweight enough to power (almost) the entire mobile web.
I thought that WebKit was based upon KHTML. Was it different enough upon initial release that you would consider it "building a new rendering engine"?
It's goal was "bug-for-bug compatability with IE" so that you could run Mozilla / Netscape on websites that weren't updated / weren't ever going to be updated.
KHTML / Konqueror was effectively "strict-mode-only" not caring (as much) if sites broke, but implementing things "as sanely as possible".
Firefox was Mozilla with a sane UI on top of it.
Mozilla UI was trash because goofballs in suits kept ruining it by pushing for "site-specific-themes paid for by advertisers" which caused the "chrome" to be incredibly buggy / slow / etc.
WebKit / Safari was Apple delicately picking up KHTML, making the "hard" decisions to implement some things poorly / hackily / different / more quickly than the "purist" open source KHTML volunteer developers had envisioned.
They actually handled it overall quite well, as opposed to their other forays into open-source land (kernel / darwin, cups, etc).
WebKit is excellent now because it didn't have to start with that bridge step, and had speed / correctness / isolation as a focus from the start. So even though WebKit is the current "leader", it owes a lot to Mozilla for doing the hard grunt-work that allowed it to take cover behind the big lumbering dinosaur and come out unscathed on the other side.