Unfortunately I have no insights how Allegro managed to stay ahead of the global heavyweights. Probably combination of polish language UI and catering to specific needs of local market.
Currently while both eBay and Amazon have sizeable presence in Poland, the Allegro.pl is still the go-to place, and seems to be able to hold its own pretty well, up to and including expansion to neighboring CEC countries. Even the Amazon fulfillment center that opened in my city recently is mostly geared to handle the German market rather than polish.