Why wasn't the Boeing 737 grounded in 1991 after a crash's cause remained unknown?
If the cause of the accident can't be established, it can't be established it's due to a flaw in the aircraft itself, let alone a flaw in the aircraft design that affects the entire fleet. 

For example there was an accident involving an aircraft where a careless maintenance worker had installed the wrong screws to refit a cockpit window. In your scenario the entire fleet would have been grounded instantly after the accident, pending investigation.In this case they found the cause of the accident after a few weeks, but in other cases it could take months or years and ruin not just every single operator of the type but the manufacturer as well.

Think MH370 for example. That one went down several years ago now and no wreckage was ever found to investigate. In your world that'd mean that every single Boeing 777 the world over would have been grounded now for several years and probably will never be allowed to fly again because it's extremely unlikely they'll ever find the wreckage, let alone in a state that will allow the determination of the cause of the crash.

In the years since, there's been 1 other crash of a Boeing 777, and that one due to external forces (a missile fired at it by the Russian army) completely unrelated to the aircraft itself. 

Now, if there'd been a number of unexplained accidents due to mysterious causes in short order across operators of the same type, it's quite possible that operators, manufacturers, and regulators would step in and decide to suspend operations of that type pending investigation. But a single accident? No.