The reason is 100% because of safety.

Say you are removing the positive terminal with a wrench (before removing the negative terminal), and that wrench hits ANY part of the vehicle whatsoever (the engine, A/C compressor, P/S pump, a pulley, the chassis, the hood, the frame, a cable, WHATEVER), it WILL short out because that which it hits is grounded. This is VERY dangerous because, with the amperage a battery puts out, it is likely the wrench become WELDED to the surface which it hit - causing a short circuit in the battery you can't stop in time. 1 of 2 things will happen if you don't get that short rectified (usually within 1-2 seconds):
1) Your ground cable will melt or catch on fire
2) Your battery will explode

Don't risk it.

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Back in the saddle - 1972 280SE 4.5