In an earlier comment, Larry Osterman described why Windows 3.0 was such a runaway success. He got a little of the timeline wrong, so I'll correct it here.
Windows 2.0 did support protected mode. And it was Windows/386, which came out before Windows 3.0, which first used the new virtual-x86 mode of the 80386 processor to support pre-emptively multitasked DOS boxes. The old Windows 2.0 program was renamed "Windows/286" to keep the names in sync.
The three modes of Windows then became "real mode" (Windows 1.0 style), "standard mode" (Windows/286 style) and "enhanced mode" (Windows/386 style). Amazingly, even though the way the operating system used the processor was radically different in each of the three modes, a program written for "real mode" successfully ran without change in the other two modes. You could write a single program that ran on all three operating systems.
And then Windows 3.0 came out and the world changed. Sales were through the roof. I remember that some major software reseller (Egghead?) was so pleased with the success of Windows 3.0 that it bought bought every Microsoft employee a Dove ice cream bar. (Even the employees like me who were working on OS/2.) I was sitting in my office and some people came in with a big box of ice cream bars and they handed me one. "This is from Egghead. Thank you for making Windows 3.0 a success," they said.
It was a strange feeling, getting a thank-you for something you not only didn't work on, but something which totally destroyed the project you were working on!
[Raymond is currently on vacation; this message was pre-recorded.]