The simple explanation is that glue logic and wide busses have mostly disappeared from contemporary electronics. Now you have a smattering of peripherals with point to point links leaving board space for fills that would have been pointless attempting with pervasive Manhattan routing on the outer layers.
Solid fills also had a propensity to warp boards, requiring hatched patterns to relieve the imbalance. That constrained their use to boards with sufficient free space to maintain connectivity of the fill areas. PCB manufacturing has improved enough to minimize this concern.
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