Unfortunately, most have never installed an OS and so have no frame of reference of how difficult it should be. The majority just turn on the machine and the OS is already there, so for them, anything more complicated than that is already harder than what they expect.

Also, hardware compatibility. There is no easy way to know how compatible a given machine is. If the common user installs Linux and runs into compatibility problems immediately, then that will make them think Windows is better, because it doesn't have those issues.

For Linux to reach mass adoption it needs to come pre-installed on machines or else it's just too much to ask from the masses. Also, never underestimate tech illiteracy.

It is true that Windows is harder to install than Linux, but most don't install Windows: It came with the machine.