There are a few reasons, but this one normally gives you an indication.
Ok, lets say you have a machine with 8 axis and each axis needs a servo, that is not too large.
you can get 8 AC servo motors and driver and you will be paying more because each driver has its own power supply.
with the DC option, you will have 8 DC servo drivers and a large power supply.
Also DC servo motors are typically low voltage DC, so the current is higher, so typically the motors dont go too big (under a KW)
Personally, i would not even consider a DC servo system, unless you know why you need it. AN ac servo system is typically a better option.

Quote Originally Posted by Agathon View Post
OK, that's a very concise reply, but why buy a DC over and AC and vice versa?