Its because, take for example OS X.... Hmm out of the box is capable of pretty decent audio and midi for both professional and consumer applications.  No custom kernel, no need to worry about Flash working while you are running your pro-audio application.

It took you half a day, I could probably do it in a few hours at most.  But I also have more than my fair share of knowledge on Linux administration and specifically how to set up a realtime audio system for professional use.  Most people going into Linux do not know that, and that is why it is a mess.  There are distributions out there, sadly probably the best known being Ubuntu Studio, that by default out of the box do not provide an experience anywhere close to being ready for low latency performance.  You have to disable this sound system, and that sound system, and install Jack, and then get Flash working with Jack(Iffy at best).

It isn't that Linux isn't capable of being used for professional audio, far from it.  I did for a long period of time and the only reason I still don't is because I had a motherboard blow on me and haven't had the money to replace that machine.  It is because in order to get a basic experience of decent performance for professional audio applications and still not cripple most of the consumer applications out there, you have to dig into system administration that most users are not willing or capable of doing.
 

Yes there is.  Mac I install and get going right then and there, no worry about xruns because I don't have realtime preemption capable kernel, or I don't have /etc/security/limits.conf correctly set up.

Look I am all in favor of LInux, but people need to realise how these things affect newcomers to Linux as well, and if you can't see the reasoning behind why people say these things, then there is other issues to sort out.