People shop for quality and value. Chipolte's advertising and product emphasize that. Another one with many healthy choices is Panera...another long line at busy times. Ever heard of DeBella's...fresh baked breads, good ingredients and preparation. Yep, crowded.
What's my point? Many people, as they have to think of where to cut back, will absorb a small price increase if there's value and quality.
Places like Chipotle often sell to “groups” (co-workers, weekend sports teams, college students, etc.). It’s also easy to “nudge” the pricing if their peers continue to go there for lunch (”What, you aren’t coming?”).