Made more sense to me. Copper is almost never used for awards and has limited (but still some) value, thus it made sense for a starting point. Bronze, silver, and Gold are all awards and generally recognized in respective values so they were things to aim for if you started from copper, to work your way up to bronze, then silver, and ultimately gold. Finally platinum is generally recognized to be even more valuable then gold (and unlike copper-gold league its symbol had a picture on it instead of just bars making it unique) and thus was this extra supreme league for the best of the best to strive for.
The new system starts you in bronze by default, taking away the value of bronze as a reward, has gold in the middle which doesn't make much sense for what is generally considered "the best" award, then has both plat and diamond, two materials whose values in relation to each other aren't necessarily common knowledge. It may be because of my own ignorance but if you'd asked me prior to starcraft 2 which would be a higher prestige, to get a platinum award or a diamond award I'd have been unclear and probably have guessed platinum. Further, both plat and diamond have unique pictures for their league symbol, again adding ambiguity to which is better as well as just taking away from the "special" factor plat originally had.
So yeah, I'm not going to start a petition or anything to get it changed back, they can call the leagues whatever the heck they want, but I personally felt the original system with copper and no diamond was the superior one.