Probably because too much would get broken. That is sadly very often the (even more sadly, correct) argument against doing things right.
And most (older) browsers ignore unknown attributes anyway.
You're right to say that older browsers would ignore the unknown attribute, the problem is that having links broken might degrade that site to a point where it isn't usable.
Actually I'm tempted to write one to get this functionality with current browsers.
[edit] As Oscar Wild put it: The only way of getting rid of a temptation is to yield to it.