I would guess it’s that bright yellow cheez-wiz-type of liquid (the kind you’ll see in the supermarket labelled QUESO-STYLE CHEESE) that’s vital to the existence of Tex-Mex restaurants everywhere.
Fast food Mexican was too hot? I’m a lightweight when it comes to hot food and I can’t imagine anything from Taco Bell, Chipotle, Baja Fresh, et al ever causing more than a mild tingle on the tongue. Even the “fiery” hot sauces taste like tangy tomato sauce and nothing more.
All bets are off if you go to an Indian or Thai place though. When they say hot, what they really mean is thermonuclear. I once almost welded my tongue to the roof of my mouth for foolishly believing I could handle something explicitly marked Hot at a Thai restaurant and after I stopped crying, I ordered the mildest looking thing on the menu and ate it while wincing the entire time.
No chipotles either. Worst burritos ever. I swear, they’re just dirt wrapped in newspaper.
OK, the Evil Captor family is composed of sissies. 
You can get the mild salsa, you know. It’s not spicy at all. Just tomatoes with some cilantro.
I know the Chipotle the OP probably went to (site of the worst case of food poisoning I’ve ever had…those things are not fun when they forcibly exit your body at 3 am via 2 exits) and there is really no excuse for a lettuce shortage there. They could walk across a small parking lot to a store that, yesterday at noon, was overflowing with lettuce. Or they could cross the street and go to their choice of two huge grocery stores that also have an overabundance of lettuce. That being said, the Taco Bell I frequent at my mall (a parking lot away from the above Chipotle) was out of tomatoes the other day, and also out of containers for their salads…so he picked one up off the floor from where it had fallen and tried to use that…while I was watching!