"When there are locations where a lot of people already cycle against the flow, why not make it legal?"
Absolutely, yes. They should. It doesn't mean that it's not currently an illegal act, and in some places it annoys the residents. Got to balance priorities, look at why cyclists use that route, if there are alternatives that could be made more appealing etc. It's not black and white.
"There's another 'informal' contraflow on Maryfield Place which pretty much works"
Informal = illegal. Otherwise law-abiding cyclists. Etc etc.
"I don't really understand the fierce opposition here"
'Fierce'? Practical. Understanding the needs of others. Understanding the needs of cyclists. Offering other options and solutions and querying without blind acceptance. All of those perhaps. But 'fierce opposition'? In this thread?