What happened with the brand "Mitre", they had the best balls between 1970's and 90's...?

They are still around, even making cricket balls. Their web site is http://www.mitre.com/ They also make witches hats :)

If your question has to do with why Mitre do not supply official balls to the FIFA world cup, etc, IMHO, the other manufacturers are paying a lot more money to be the official supplier, OR Mitre does not feel spending that amount of money is justified.

Edit:"We are the offical ball supplier to the italian rugby federation (fir), who play with the max 460 Ball with g-spin™ technology." From http://mitre.com/rugby/index.htm

I am sure Mitre still make balls in association football and rugby union.

The best balls eh?

Sometimes they've got the balls, sometimes they don't. It's a shrinking business.