Microsoft has demonstrated repeatedly to it's customers that their software has low quality. People still stay with Microsoft, though, because there's a learning curve associated with change.

The "learning curve" argument is, for the most part, just an excuse. After all, you could always go with a Mac and have essentially no learning curve at all. I think it's mostly a problem with intellectual laziness (people don't like to take responsibility for significant decisions) and ego identification with the decisions they've already made (people don't like to admit they're wrong). There are other factors involved, of course, but I think those are the biggies.

I have run across a veritable cornucopia of excuses for a refusal to consider other alternatives. One of my favorites is the technology "investment" excuse: if you've already spent $30,000 on your Windows solution, that somehow seems to justify continuing to spend $600 a month on software maintenance and licensing rather than "throw away the investment" to go with $2000 in setup fees and nearly zero-cost upkeep for an alternative, open source solution that you might even discover better suits your needs if you took the time to evaluate it. Of course, that's not to say that changing technologies is always the best option, but hundreds of thousands of businessmen will never know because they never make the effort to find out. So it goes.

It's a bit like politics, too. People continue to vote for a candidate from one alien lizard clan to keep the candidate from the other alien lizard clan out of office. Humans just aren't programmed to be comfortable with choices any more complex than binary decisions, and they even try to avoid those as much as possible; if a decision has already been made, it reduces the number of decisions they have to make in the future, and woe betide the guy that tries to convince them otherwise.