I think it's primarily a combination of "use the right tool for the right job" and cost.
Don't flame me on this these are rough prices...
Mac 2k's $15k
Mac 700's $10k
Mac 600's and 550's $5k-$7k
If a Mac700 has everything you need for the specific purpose you don't need to spend the extra $5k for a
Mac 2k? There's no reason to get a Mac2k or a 700 if you don't need
CMY for a specific
effect, you can get by with the 600's and 550's instead. So budget vs.
effect needed plays a huge
role.
Also if you get into the specific details of what each can do it quickly becomes like asking the question why do you need Fresnels, Elliposidals, and
PAR's. The difference between a
Wash and a Spot is a huge difference. There's also a big difference in Lamp
intensity. Then you add things like single or double
gobo rotation,
CMY or filters, animation wheel, shutters... they are all very different.
The real challenge is being the person with the skills to design the show and know which specific instruments you need for which
effect. That takes talent.