Buses don't take the Goa route because that would add more time to the already long 18-20 hours journey time via Yellapur-Belagavi. There's no other reason. We're talking about Mangalore/Udupi-Mumbai travelers here, going via Goa or not doesn't matter in this context.
The demands have been made not by railfans or rail activists but by passengers themselves. These are the ones who will utilise the service at the end of the day.