What were some of the query features that caused you to choose Elasticsearch over pg_trgm, because pg_trgm supports regular expressions out of the box.
As for why we were using ES for the rest of search: we were using it for things like language stemming, matching terms with "slop", things like {'minimum_should_match': '3<67%'} (require exact matches if 3 or less terms, 67% if more than that), searching _all to match anything in the document, etc etc. I think a bunch of these (maybe even all) you can do in PG, but it was way easier to get going with ES.
ES is also distributed, which makes scaling up and doing maintenance a lot easier - a bunch of times we just threw new nodes at it and remove old nodes that had gone bad.