More specifically, the demand for fiber didn't come - but the demand for bandwidth did!

Those companies were building out fiber like it was a legacy telephone network: needing a massive amount of individual wire pairs to carry a massive amount of parallel transmissions. In reality the bandwidth per channel was rapidly increasing due to better tx/rx equipment, and with the rise of WDM it was suddenly possible to carry not one channel per fiber pair but several dozen.

When you were laying fiber on the assumption that a single long-distance pair was going to carry 1Gbps - and therefore laying hundreds of fiber pairs in a trench at once - it is quite a big upheaval when that single pair can now carry 100 channels of 10Gbps each. Your brand-new expensive fiber trunk can suddenly be replaced by a single spare pair some utility company had lying around already.

Turns out laying fiber isn't a great business when the capacity of existing fiber is doubling every year or so!