I think you could make the somewhat tautological case that IE 6 is not bad enough to make many people use something else or that php is good enough to make many people choose it.
Defining good is always tricky, but being chosen by many people when other good choices are available is not a bad test for certain definitions of good.
Choices may not always be %100 rational. They are usually not random.
But IE6 is bad enough to make people use something else, and PHP is not good enough to make many people choose it. Who purposely downloads and installs IE6 these days when you have Chrome/Firefox/IE8 available? And who writes a new webapp in PHP?
Most people seem to miss the role that time plays in language choice. PHP's very far from the best choice for webapps now, but it was the best choice when it was invented in 1995, and even for a while afterwards (such as when FaceBook was created in 2004).
Nobody purposely downloads IE6. That isn't the point though. If they buy a computer (maybe this isn't true anymore) and go to Google using it they don't go "something's wrong." They just keep using it.
Defining good is always tricky, but being chosen by many people when other good choices are available is not a bad test for certain definitions of good.
Choices may not always be %100 rational. They are usually not random.