it was because they felt people would stay in lvl 50 content to hit lvl 60 fastest and they want people to level mostly in ishgard.
they did say they will add exp to the content eventually.
here's the quote and link for source
http://www.bluegartr.com/threads/125...dia-Event-5-19)
With the way the hard and expert dungeons from A Realm Reborn will be kept separate [[from Heavensward content]], will it be possible to level up there?
Yoshida: It was a tough decision, but we decided to leave it so that you will not obtain experience there at the start of Heavensward. As a development team, we believe that moving on past level 50 should happen on the Ishgard side. Because the new dungeons have a different pacing than the ones from A Realm Reborn, we were also afraid that people would gravitate towards the older instanced dungeons for more efficient experience.
After the situation settles down from the release of Heavensward, we might change things around by assigning some experience value, but that remains to be seen.