Why is it that they are less influenced by competition and still continue to be available while a fistful of non-IE based browsers eg Firefox, Opera, Netscape Navigator keep trying to survive every moment!?
Answers:
The non-IE browsers out there are surviving due to offering a better experience in terms of safety, speed, better features and honoring the W3C web standards. If IE was not pre-installed in Windows, I'm convinced you'd see alternatives to IE reach much higher percentage of the share. Lots of users are not aware of the alternatives as they find IE to do what they want, but I believe are likely to change if they have their system compromised in any other way (infected by virus, phishing scam).
I'm surprised people are still using IE, when Opera and Firefox are by far superior. Personally I prefer Opera for the speed, small size and the best out of the box suite of functionalities, but it all boils down to personal preferences.
The fact that so many IE add-ons exist show that IE is lacking in many features. Further, Firefox and Opera are doing a lot more than just surviving, but are furthering the state of browsers. Since about IE 3, there's been little innovation on the Microsoft side. Without Opera, nobody would have tabbed browsing. IE 7 is a move in the right direction due solely to keeping up with the competition.