what happened to Nissan? why Toyota is far above it in vehicle sales? when this happened and again why?

My experience with Nissan goes back to Datsun's. As in 1972.

My brother bought a brand new Datsun 510 and I bought a brand new Corolla (1600).

Now he was happy with his Datsun, but maybe that was because he traded in a Fiat for it.

I drove both his Datsun and of course my Corolla.

In all honesty the Datsun was no where near as good a vehicle as the Corolla.

Move up a few years and Datsun rebranded itself as Nissan. The reason is simple, Datsun had gotten a bad name and they did it in hopes that they could get a fresh start in the US market. In Europe Datsun seemed great and reliable when compared to British, French, Italian and even German products.

What I am saying is Nissan is paying for marketing an inferior product. They tend to be more cutting edge technology, but that comes with the price of not having the bugs all worked out of the product.

All of that said, I buy Mazda's or Subaru's when I am in the market for a car and will consider a Ford. But not a Nissan or a Mitsubishi product. Never GM or Fiat!

Nissan has always lagged behind Toyota and Honda in sales and quality even before the merger with Renault.  The merger made things worse.  

Nissan went downhill when it merged with Renault.  Why they merged with a car manufacturer that have not sold any cars in the USA for a long time is a case study for MBA students.

Nissan has had too many quality control problems. Their CVT transmissions in particular are a bad bad design and not trustworth. Toyota hands-down builds better vehicles and always has. A lot of people switched to Nissan because of cheaper prices at every vehicle level. The Koreans are now building better cars than Niussan Motors does and I wouldn't be surprised to see them drop to 4th or even 5th place in the market.