It may just be the way your fast twitch (white meat) to slow twitch (dark meat) fiber proportion works.
I am always pedalling at less than 70. That's where I'm happy. Slow twitch zone.
In track-n-field I always ran long distance, and set lovely records. I truly s*cked at sprints. Again, slow twitch zone.
I can side-stroke and froggie-kick my way across vast bodies of water, but can't crawl or flutter-kick to save my life. Slow twitch zone.
If 70 feels good, and 90 is miserable even after giving it the ol' college try; it just might be the way the good Lord intended you to pedal.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson