Spoke to a nissan salesman about this in the summer when he told me uk wide nissan had only sold either 15 or 50 370 convertibles in the previous year, hence the cease on stock.
He said, and I agree, that the issue is outside of places like this forum no one really knows the 370 exists. It is not advertised enough. Focus is either on the nissan mass production cars or the GTR, so it has no presence or identity in the market. People don't go looking for them in the showroom, most sales come from people who walk in for something else and go - oh what's that. And there are not enough in stock in the uk showrooms to have much impact.
The roadster is a step further on the specialist scale so even fewer people want those. Less practical, harder ride, less space in the cabin. Big n/a thirsty cars are now a rare breed and dwindling fast.
Still it means the roadster is a rarer sight on the road which puts a smile on my face