But in this day, why can't the Indian Ocean be opened up to more traffilc?
Fair question. The answer is because the Muscat ATC authority is a loooong way behind the curve. They have needed a lot more controllers for a long time. I understand they are slooowly addressing this, with perhaps, an increase in numbers in the near future. Inshallah.
Many aircraft divert around the undermanned Muscat FIR due flow constraints that are there simply due to a lack of manpower/lack of will by authorities to fix.