Phase 2&3 trials are costly to run and here's hoping that we need to find the money to run 5 of them! That's at least a pipeline of ~$500-1B needed for trials.
You either do it through CR, an agreement or a buyout.
Big pharma due to failure risk - mostly let others take the punt, too risky for them. Buyouts are common as risk reduces on science (mostly post phase2), money becomes important and big phama start sniffing. buyout often occurs as they want IP which is where the value is.
But then that didnt happen with GW. much later down track