Someone will chime in regarding the minimum connection time in IAD - for both regional to international and for mainline to international.
It might be that regional to international MCT is 0.75 hours, while mainline to international is .50 hours (30 minutes) and hence why it can't be booked.
Of course, if both are .75 hours, your flight is still below 45 minute MCT.
Last edited by aacharya; Apr 10, 2015 at 5:04 pm