They likely started at level 1 so that people with less experience playing DnD or a specific class (Ally comes to mind) could learn their character as they went along, not to mention the fact the characters were all high school students.

It also meant that first time DnD players/watchers getting into it through CollegeHumour/Fantasy High had an easier time understanding the system.

Since most of the characters in The Unsleeping City are older and or have more experience, it makes thematic sense to start them at a higher level, such as in Escape from the Bloodkeep.

With the cast of EftB it likely drew lots of people familiar with Geek & Sundry, who would have more familiarity with DnD and thus better able to keep up with a game starting at level 14.