Previous versions of Netscape Communicator/Navigator were arguably superior to IE as day to day browsers but they suffered through their immediate usability and modularity. Although the Netscape browser was great as a standalone application, it wasn't possible to utilize that functionality in third party applications. On the other hand, Internet Explorer shipped with an ActiveX control which allowed exactly that ability.
Take a look at some of the applications that already use the IE control:
AOL - For displaying a web pages in the AOL client
Encyclopaedia Britannica (CD)
Microsoft Encarta Encyclopaedia & World Atlas (Globe)
Microsoft Outlook - For HTML mail
Microsoft Studio - For online help
Qualcomm Eudora - For HTML mail
Neoplanet - Another skin, this time commercial
Custom Browser - Made to measure browsers
Active Worlds -VR conferencing that allows browsing in a pane
Intuit Quicken - For reports and statements
TopStyle A spiffy style sheet editor
Allaire Coldfusion - Web site authoring and server side scripting package.
HTML-Kit - HTML development environment
There are hundreds of others apps, some commercial and probably many more running in intranets all over the world.
So what would it take for developers to use the Mozilla control as opposed to the IE control?
An API close as possible to Internet Explorer's for ease of porting
Freely available source code to allow for bug fixing, customization, etc.
A very small distributable - 3-4Mb (for a download containing just the Gecko embedding engine and not the full Mozilla) compared to the 10Mb+ required for IE
State of the art rendering - exploit the speed and standards compliance of the Gecko rendering engine
No more nasty IE license