Possibly the fact that Delp might have had his hands tied (so to speak) by Scholz contractually. Scholz (who let's face it, loves his litigation) probably wanted to preserve every element of the Boston sound, which included Delp's voice, for as long as he could.
Also, Delp may well have 'only' written (or co-written) two songs on the debut (plus two on 'Don't Look Back' and 'Third Stage') but those three records have combined sales of approaching 40 million, so he would have certainly brought in enough money (publishing, PRS, mechanicals...) to maybe feel he didn't need to work that much between Boston releases. Maybe...