I think possibly the most common reason is that I.E. is what is installed with Windows, and that MANY programs & sites specify IE as default when updating...etc. So many folks simply do NOT like installing anything or peeking "under the hood".

I just happened to early on begin frequenting a forum where real techies hang out and got reading about all the alternatives (beginning with Netscape) and why most are 'better'.

I have since found I really, really like the internal workings and add-on possibilities and configuration choices in various browsers like Opera & Firefox and several others.

(and to comment specifically on Jack's " Ain't broke, don't fix it." remark... It ain't a matter of 'being broke'! You can wash clothes in an old ringer washer, too! Or cook on a wood stove. But most of us know of better ways.
If you have never seriously tried Opera and/or Firefox..etc, I can only shrug...)