It has to do with stupid marketing decisions and poor marketing of the car.
Sunny is a very high quality vehicle. Lets compare my friends 87 Sunny to my 89 Honda Civic. Sunny had a very high quality interior---nothing vibrated or rattled. Everything that your eye saw in my Civic rattled and vibrated. Sunny had an upscale looking steering wheel and gear knob. My Civic had the kind gear knob that you found in a 77 Corolla. Sunny was as comfortable as my fathers 88 Accord. My civic's ride was extremely poor and bumpy. Sunny had a racecar like road grip. It could take a turn on 40kmh that my civic couldn't even do at 25kmh. I remember we were racing an 88 Corolla in front of president house. We won the race because the corrolla started skidding and had to slow down. My friend's Sunny, however, had an extremely tight bite on the road and won because of road grip---not of high acceleratoin. My friends Suuny was 1000cc and it could beat a Mehran (called Alto at that time). My 1193cc Civic, however, could not beat a Mehran. And the sticker price. 2.70 lakhs for the Sunny and 5 lakhs for the Civic!
People have attached an underpowered name to the Sunny. Back then they offered it in 1000cc, which was way underpowered. They still think its underpowered.
One other thing. Nissan closed their 3S dealerships in the near past leaving their customers stranded. People don't have confidence in NIssan anymore. You may buy a car today and who knows, they may close their dealerships after a month leaving you hungry for spare parts and maintenance.
I won't buy Sunny because of the second reason I posted eventhough its a great car!