It was also closed source, so nobody who found a bug could just go fix it. You had to ask Microsoft, nay beg Microsoft, to fix it for you. And they didn't.
These effects compound one another. The result is that MAJOR bugs become features.
How do you think it got from IE1.0 in August 1995 to IE6 in October 2001? (That included the IE5.5 release as well.)
That rate of development with the resources Microsoft had to get the best developers available is really unacceptable.
The IE team produced 7 versions in six years and there were some very substantial advances, including a new layout engine (Trident). They also did Mac and Unix versions, a mobile version, and a tabbed version for MSN (before iE had tabs).
IE certainly developed a lot faster than anything else on the market in the 1990s, bearing in mind that Netscape took three years to get from 4.7 to 4.8.
Safari entered the market late (2003) and still took the best part of seven years to make it to version 4.
Nobody shipped major versions "yearly".