Ok, I have a theory which goes against the grain of the British palate.
When the first Indian chefs in the UK decided on using a large pot of onion gravy for all the curries, they found it too sweet for their taste and so added lots of turmeric to balance it out.
Too much turmeric = bitterness
Lots of onions = sweetness
But, if the British public prefer their curries to be sweeter than their Indian counterparts, then wouldn't it make more sense to drop the huge amounts of turmeric rather than adding additional sweeteners like sugar, coconut, mango chutney etc...
