This article makes the point. The fungus has no quick propogation methods, as it's only spread by moving infected earth: it's simply very resilient once established.

So the last X years have been a slow motion "We know this is a problem. It's becoming a bigger and bigger problem. Eventually it will be an insurmountable problem."

The article also notes that there were a substantial number of "flex years" the last time this happened, as deforestation was used to open up new, un-infected land for banana cultivation.

In summary: it's like the IPv6 problem.

We knew it was coming, but resource exhaustion takes time.