Why do most pro photographers choose Nikon?

I have notice most of all the pros are using Nikon. Why exactly is that?

The Lens quality is the best

I disagree with the premise of your question.

Watch any major event with lots of photographers - eg the pit lane of this weekend's US F1 Grand Prix and the majority of cameras have white lenses - they are Canons! I would say that Canon have it by around 2 to 1 for formula one photographers.

Nikon and Canon are both popular because they are high quality products, with a huge range of accessories and with tremendous user backup from the manufacturer

They don't - it's about 50/50 and has been for decades.

Sony's full-frame cameras seem to be treading the same path as the Pentax LX did back in the days of MF - no matter how good the camera (the LX was a cracker) there's no room for a third manufacturer in this market.

Unfortunately, the amateur market seems to be going the same way.

At high-roller events, the number of Canon and Nikon users would be just the same. Anywhere else you'll see the difference. Most non-paid photographers use Canon while most of those who get paid use Nikon.

Nikon dSLR's are more technical in feel and appearance. You could get to most any control with the clock of a button or two even with their low-end models. It's not their thing to be user-friendly (like Canon), ergonomic and stylish (like Sony) or simply old-fashioned (like Pentax). Nikon has the tendency to release a model once they develop new technology. It's like they're always cutting edge. That's a tradition they maintained even from the old days of film.

Sorry, but you don't know what you're talking about.

Nikon and Canon, both releases high quality cameras and accessories. I my experience, I found Nikon camera software very user friendly.

Many Nikon users choose it because (I've heard) they can use the old SLR lenses on the new DSLR models.

But is is not true that most pro's are Nikon users. The two types are much of a muchness if you compare similar cameras. It basically boils down to personal preference. I chose Canon because I prefer the sound of the shutter. It's like comparing the sound of Lamborghini against the sound of a tractor. Maybe a silly reason, but it is the truth. I find the sound the Nikon shutter makes very off-putting.

I don't think you've see most pros. It's probably more like 50/50