Because it works with the brake callipers as a system. The Beetle mc does not have enough volume to work well with the single piston sliding FRONT callipers from the early 944.
The 944 has a split bore mc, 23.?? for the front, and 19.06(?) for the rear. The rears work fine with the Beetle mc because the rear portion of the 944mc is the same diameter as the Beetle mc.
If you sit down and work out the front to rear ratio, you will be surprised, the 944 is not as bad as you think.
If you use a 911 mc, then use it with 911 callipers. You need to compare the mc volume and the calliper volume between the models, not just the master cylinder.
http://www.germanlook.com/Forums/showthread.php?t=2183
The later 944 4 pot setups are a different kettle of fish, and guys here have found the Beetle MC works with those callipers. I can't comment on this setup.
Cheers
Jeremy