It would help if all aircraft had them.
Cost is an issue
So is weight
So is the design of retrofitting installation
The question is whether the penalties are worth the gain. As a general rule, the crash investigators do get to the root cause of accidents in light helicopters without the aid of flight data and cockpit voice recorders. Where is the body of evidence that shows the potential gain and weighs this against the negative sides?
Don't forget, when you say "all commercial flights" (Cabby) then you are including sight-seeing tours in an R22 or a G2.
My personal view - call for black box is a knee-jerk reaction; "we must be seen to do something". Had they been carried on this flight it would still have crashed, and I have difficulty to see that we might know more than we do now about why it crashed.