The thing to understand is that Microsoft's main customers are larger business and government, who have large installations. These organisations tend to be very conservative. They don't like to just deploy software/updates, rather they usually put them on test systems and check that the updates don't affect other software.

I.E is also a lot more integrated into windows then the other browsers. So chrome largely just needs to test that their browser works, Microsft needs to test that that the browser works one each version of windows (Vista/7/8) but also all versions (Pro, Home Basic, home Premium etc), but also for all supported languages (Dutch Windows Pro, Dutch Home Basic etc). In addition they need to test that an I.E update doesn't break other software like installers, Office, popular non Microsoft software that makes use of I.E.

Given that their customers will probably take their time updating it makes sense for Microsoft to very carefully test any updates as a hot patch released 10 hours after a bug is found probably won't be widely deployed any faster then one released 10 days later. And 10 hours probably isn't long enough to test all the software that relies on I.E in some manner.