Why is CTU (Chengdu) so underrepresented by OW?
 

Only Cathay and Qatar fly to Chengdu. Cathay's flights aren't conducive to ex-US connections and places you on an Air China codeshare.

It would be helpful if Cathay's flights were more layover friendly and JAL flew there.

A connection from the US to KA is typically 2-3 hrs (depending on daylight savings) and does not involve a codeshare.

e.g. DFW-HKG-CTU on AA125/KA824 is 1:45, LAX-HKG-CTU on CX881/KA820 is 2:45, SFO-HKG-CTU on CX893/KA820 is 2:40.

What sort of a connection time do you consider "friendly"?

A lot of ex-US flights are red-eyes for Cathay on purpose to get passengers into HKG early for connecting flights onwards. They're pretty friendly IMO.

A codeshare with Air China still gets you OW tier points / mileage. Honestly the difference in hard / soft product on CTU <> HKG isn't so different between Air China and KA -- I've flown the route on both before.

Not for non-MPO (non-CX) members.

What happened to BA's flight to Chengdu? I remember seeing a documentary which focused on the fanfare of the launch of that flight. When was it scrapped?

Mainland China is simply not a friend of OW.

BA Chengdu service lasted 3yrs
https://www.theguardian.com/business/2016/oct/24/ba-scraps-service-chinese-city-airport-expansion-heathrow-chengdu-decision-runway

Documentary about BA launching their (now scrapped) Chengdu service: https://youtube.com/watch?v=vTaPtkTwniU