Probably because its brakes are designed to stop something like 15 thousand pounds.My 2002 F-350 powerstroke dually hauls to a stop vastly better than the disco1 does. ;-P
D1 brakes aren't bad, but nothing to brag about. I find it funny when people complain about RRC brakes and want to swap the entire setup to D1 vacuum servo - RRC brakes are vastly superior, and [when the pump works] they can be used with a dead engine.
That all said, when you upgrade the calipers to 110 front and rear (even with the same rotors), and fit a proper master cylinder (even to the same vacuum servo), you should see a great improvement.
To give you an idea - my prop valve seized about two years ago, and I drove the D1 on 31.5" tires for a year without even knowing I had no rear brakes at all.