There is also another factor, usually ignored. Some software packages become eventually de-facto standard, even to the point of becoming part of the colloquial language: "a top-model photoshoped her picture", "you don't know anything about stackoverflow, then google it", "please tell me what you think and don't just copy-n-paste what you heard from the TV yesterday", or even older such as "hoover the floor". At the moment you start a business you'd just buy them without considering alternatives.
Additionally going toThere is also the "Let me go Back" principle (I'm such a fanboy). Choosing an alternative solution might cause problems. (Personal true story follows) Once I had to fill an application for a company that (although was not a Microsoft only shop) had an application in MS-word with it's forms. OpenOffice corrupted the document after saving it. My application processing got delayed and almost lost the deadline. I remember having to find a friend with a windows PC in the middle of the night, when I saw my document there, I realised that it was unreadable for the HR-person on the other end... I asked the HR at some point why do they force me to use a windows-based PC and product. They were very polite to say to me that I was the first one that ever complained...
update: another story after the comment of @TrevorPowell, this time from the mid-90s. It's long and boring.
Back then we used floppy drives (1.44 MB per disk). This meant that for any file that was bigger than 1.44 Mb, you needed two disks to store it. But to get it to someone else safely you needed 4, the reason why was that FDs (acronym) used to fail a lot, so a second copy should be available. For bigger amounts of data you needed ZIP drives (100MB per drive). Now if the 1.46 file could be compressed to something less than 1.44, you magically needed only two (one and backup) disks. That's why compressing was so popular. At some point zip was bundled in Windows, at that time I needed to print a large Quark XPress file which would fit into 6 floppies compressed with... RAR. I remember when I showed up to print shops with 12 floppies as a kid, to get an offer, most shop owners threw me out: Where is the zip-drive (100MB). For them it was zip or one floppy. Few of those that would look at the floppies, stopped talking to me because they did not knew what a RAR was or believed that a R00 (extension) was a virus. Once one asked his "IT" guy to come, he knew what was rar but would not install it in his machines. You can imagine that after that, I went to a friend with a... CD-writer (very rare commodity then) and we burned the uncompressed file to a cd-rom... That was the most acceptable way to do it back then, it did the trick.