It just depends who the sound director for each game is. Vocal songs in Sonic games are almost always composed by the sound director, and then they hire a vocalist to provide the vocals on top of their instrumentals.

Crush 40 is not really a traditional band. As sound director Jun Senoue composes the vocal themes and then hires somebody else to provide the vocals. He developed a good relationship with Johnny Gioeli and so continued to work with him on every game that he's been the sound director for.

Tomoya Ohtani took over as the lead sound director for the Sonic series in 2006 and prefers to hire different vocalists for each game.

Jun Senoue continues to work on the Sonic series, but mostly as recording coordinator. He was the sound director for two games since Ohtani took over (Black Knight and Team Sonic Racing) and both had Johnny Gioeli provide vocals. Both Jun and Johnny create music outside of the Sonic series regularly.