Well technically they did use the original voice actors from G1. Except for Arcee who was voiced by Kawanami Youko (whereas I think she was voiced by Katsuki Masako in 2010 and another seiyuu in the Japanese dub of Transformers The Movie).
But perhaps you meant the original English-language voice actors in a dubbing of this series. I've heard that they actually did plan on dubbing all the Japanese G1 series over with the Anglophone voice actors in the early 1990s. Then someone decided to "improve" on the Japanese series by adding intermittent computer graphics known as the "Cybernet Space Cube." But then someone decided it would be better to retcon the old episodes in the same way first before introducing audiences to the Japanese episodes dubbed. So that's what they did. This show was called "Transformers: Generation 2." It flopped miserably so they never got up to dubbing the Japanese series.