Route ,day of week and time of flight are important factors to consider when hoping for your EUA.I almost never EUA Transcons or IAH-EWR so I attempt at ticketing time to upgrade using miles or a Y-up. It is also a difficult EUA when you book a flt with a 735.
Thed poster seems to have a good question concerning the discretion of phone agents to EUA for reasons other than Y-up or a mileage upgrade.