Avocados weren’t really marketed at all until the mid-90s and were also shunned by some due to high fat. It was a niche food almost completely considered a side for mexican cuisine. It has been expensive outside of growing areas for a while, hence the jokes about asking for guac at Chipotle and always being warned it costs another $1.39. I’d guess part of the expense is that it has a relatively small window between being unripe and too ripe.

My understanding is also that it can take 20 years from pit to fruit-bearing-tree, so supply will tend to lag behind demand (which has grown globally.)