I know people will say it's because it's cheap. But the odd bits I've had from Primark in the past have fallen apart, looked crap or lost their shape after very few wears. So to me it's false economy, as I end up having to replace it soon anyway.
And it's not even that cheap anymore. I visited a shop recently to have a browse and many of the products are not much cheaper than other high street shops that are better quality. Supermarket clothing is often cheaper and lasts longer.
I've never understood the big fuss.