How that market share was acquired was also a key factor and where the long series of lawsuits began. It took years for the various state justice systems to batch things up and bump it to the federal level.

The starting point was Microsoft forcing OEMs to buy a copy of MS-DOS and/or Windows for every computer that went out the door. Even when said computer didn't ship with any Microsoft software. This was also the foundation of the EU case against Microsoft, from a complaint by Novell originating in 1993.

Being able to set default programs only came as a resolution during the tail end of the lawsuits, due to evidence that Microsoft on both business and technical levels was abusing it's OS monopoly powers.