Because they
want to make money? 
 I must be very thick today, so you'll need to explain.  Said vendors
 already supply both OS X and Windows versions to all purchasers.
 Which is actually used is entirely up to the customer. 
 
While I'm sure it is possible to make money in Linux software, I
can tell you from personal experience that there is a strong
perception that "all things Linux should be free" - I sold cross
development tools for many years (still do actually), and I've
been asked about Linix versions many times - I don't think I ever
was asked if I'd sell Linux tools, or what they would cost etc...
But many times I got asked "how about free versions for Linux?"
... I never developed linux versions of my tools.
Regards,
Dave
--
dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: 
www.dunfield.com
com             Collector of vintage computing equipment:
                
http://www.classiccmp.org/dunfield/index.html