The healthiest lettuce is, of course, the one you will eat
Seriously......there are many people, especially Baby Boomers and older, who didn't grow up on farms and thus never saw any lettuce other than iceberg. It was the only kind available in the stores and served in restaurants when we were young---I don't think I even tasted Romaine until I was in my late teens---so that's what a lot of us 'old folks' prefer.
For myself, I still like iceberg lettuce better than any other kind, but I've learned to enjoy other types of greens in my salads, including spinach. It's been a steep learning curve; I was not brought up in a veggie-loving household, and I loathe spending time in the kitchen so it's a good thing that prepackaged salads were invented!