I have to wonder if it doesn't have something to do with the way he is mentioned along with the other Buddhas and the Bodhisattvas. Of course it can't as simple as that, but it may be related. My take is:
When listed with the Buddhas and Bodhisattvas, the title "Buddha" is used to be specific. That then begs the question, "Why not Gautama Buddha?" I've noticed that people have occasionally referred to him as "the Shakyamuni", which points out that "Shakyamuni" can serve as an honorific and attainment-based title while "Gautama" cannot. "Shakyamuni", however, still remains specific to an individual. "The Bhagavan" does not say which bhagavan and "Gautama" is referring to an entire family. Shakyamuni is the best of both worlds.
This thinking is quite a bit on the fly, so I hope it proves helpful.
"Once delusion is extinguished, your wisdom naturally arises and you don’t differentiate suffering and joy. Actually, this joy and this suffering, they are the same."
— Chinese hermit, Amongst White Clouds