I'd argue the biggest issue with concretely defining intelligence is that any attempts end up falling in two buckets:

1. "Too" Broad, which raises uncomfortable questions about non-human intelligence and how we as humans treat them (see: whales, elephants, octopuses/cephalopods)

2. Too narrow, which again raises very uncomfortable issues about who and who does not qualify as human, and what we do with them.

Put in other words, it's more an issue of ethics and morals than it is definitional.