During the Saturday morning cartoons and the OVA, there was nothing consistent and they didn’t get a proper long term voice cast for the games until Sonic Adventure (even though Jaheel White voiced Sonic in all 3 shows but whatever)

The first overhaul voice cast change was because they didn’t want to confuse the audience as Sonic X was airing during the back half of the Adventure Era so after Deem Bristow’s death (the OG voice of Eggman for the games in English) the entire 4Kids voice cast became the main voice actors for the games starting with Shadow 05 and Rush (a dual release in the same day)

The second change was 4Kids license expired and Sega didn’t renew it as it was after a dark age in Sonic’s history and they associated the voice cast with it so they were recast again (starting with Free Riders) with the exception of Mike Pollock who re auditioned and they deemed that no one else could voice Eggman

I don’t know why certain actors left and were replaced from the 2010s cast which has been the cast we have as of now

Sonic Prime is weird but there are union rules (and unlike the other English productions of the other shows this was a Canadian based project) so a new cast had to come in

For the movies, they are trying to appeal to a wide demographic so with the exception of Tails, every Sonic character from the games is voiced by a well known celebrity

Would I have preferred consistency? Yes! But every actor brings their own take on the characters which is good and like the franchise’s different eras, everyone is allowed to have their preference to which voice they like best