Because the reasons for changing, not that long ago, were - in my opinion - spurious.
You have the advantage of me here, because my historical knowledge on this is poor. I have long understood that the routesheet is advisory and the requirements are simply to visit the controls in the prescribed order and within stated time limits. I take it that, once upon a time, the rules (or perhaps only the spirit of the rules?) required that riders followed strictly the route set down in the organiser's instructions?
[nb I think we're in the wrong thread here]
The history is actually very poorly documented.
Up until and including 1999, there was an AUK regulation which said
"
Routes should be interesting. Riders must keep to the alloted route and should they leave it must retrace to join the route. ..."
The 1999 AGM changed that, and subsequently there is just
"
AUK and organisers will be responsible only for indicating or agreeing control points to confirm that a participant has completed a predetermined distance ..."
The minutes of that AGM (currently hard to find, temporarily lost in the AUK website migration, try
here) don't document any discussion of the change, which was part of a wide-ranging regs review introduced at that time.
So the reasons for/against are a bit lost in the mists, but as I recall it was simply recognition of the fact that mandatory routes were not being enforced anyway (organisers don't have that sort of manpower), so why have a regulation that can't stick?
It's true though, that the change has had some unforeseen (and undesirable) consequences, including the ridiculous proliferation of info controls.