Combined with the fact that he was originally gonna have Gohan take over Goku's place and let Goku stay dead, but editors and "popularity" caused him to change it.
Now Gohan does nothing, but be a father. The new movie seems to show him being better, but these characters just get stronger for no reason now.
Originally, with the Cell arc from what I understand. He wanted it to end with Goku's death, and Gohan taking his place as Protector of Earth. When he was convinced to continued with the Buu arc, Gohan was the main character at the start, but actually focusing on Gohan as a character he realized that, Gohan is a scholar not a warrior. He played with Gohan alone for a very short time before bringing Goku back, but didn't put Goku back as the protagonist till around the fight with Super Buu.
So, it wasn't that he was forced to change back to Goku while not allowed to explore Gohan. Its that he tried Gohan as the hero didn't think it worked and eventually went back to Goku...with likely some suggestion from his editors.