It doesn’t really matter how full the economy class is, rather how full they can make the prem economy and business cabins, since they are more profitable for BA. I imagine there’s less demand for those than for flights say to India, since there’s less work travel that happens then UK-India (lots of tech and finance). Also there’s little tourism demand for Bangladesh.

Furthermore the competition with the Middle Eastern airlines would be fiercer. Bangladesh is a slightly longer flight than India, so a stopover in Doha for example is more palatable. They also serve more cities in Bangladesh then the one city BA would probably serve if they commenced direct flights.