It may be because for many use cases -- probably most, statistically, although that's just an intuition -- what you're really looking for is a configuration data format, not a full-blown language. You need key-value pairs, probably nested, and... that may very well be it. It may not even matter if the data format can explicitly specify data type.

Dhall and Cue are no doubt excellent and amazing for complicated use cases, but there are a lot of use cases where using them is akin to taking the VTOL jet out to pick up a few things at Trader Joe's.