No one "spoke that much" back then, but they still used the same voice crew throughout the series. To just drop them now is wrong.
Looking at the IMDb pages for the old voice actors for much of the WarioWare cast, they don't have many voice-acting credits outside the series, and many of these other credits are just in smaller parts, i.e. parts only listed as "additional voices." Heck, most of them have more credits under other jobs working on games. This is understandable- because there wasn't much speaking in the older games, they could get away with casting people who were either less experienced in voice acting or who weren't even in the field to begin with, both because the jobs were less demanding and so they didn't have to pay more professional VAs' higher prices. But Gold is different- because it has fully-voiced cutscenes, it only makes sense to hire professional voice actors for the roles. I understand not liking it from a fan perspective, but if the voice-acting was given by less professional people, it would probably come across in the performances, which would negatively impact the people who may not have been so attached to the voice actors they used.
Take the Ace Attorney series for example. For the first few games, the only voice clips in the game were Phoenix Wright's iconic "Objection!" "Hold it!" and "Take that!", as well as the objections of each games' prosecutors. These dub voices were also provided by people who worked other roles in game development, because all they had to do was get in the recording booth, shout three different lines max, and leave. Then Ace Attorney: Dual Destinies came out for the 3DS, featuring some other lines spoken aloud on special occasions, as well as fully-voiced anime cutscenes. They recast the returning roles with new, professional VAs, because a larger-scale project requires more experienced workers, no matter the field.
Sorry for giving an overly-long explanation behind it, and none of this is to say that you have to
like the new voices. Just thought I'd give some insight as to the "why" factor.