Why was the M90 discontinued?  Maybe 50cal will chime in here.

I bought an M90 from Barrett back in the 90's.  I had to return it due to issues with it not firing.  Ended up, Barrett replaced it with another gun.  Hated to see that one go because it was serial number 69 .

I've had no trouble over the years with the replacement, except for the fact it seems to have a really short throat.  You have to watch your loaded length.  

Just wondering what the updates/improvements to the M95 were.

Also, how many M90's were made?

Here

Read my first response in this referenced thread.

There weren't a lot of M90's made. If memory serves me right it was around 200 or so. Maybe a hair over.
They were difficult to build. They were hand fit one at a time. Test fired, then stripped and parkerized. They looked unfired when they left the factory.