The first is that restaurants always want there to be at least one option that isn't red meat; oftentimes this means both chicken and shrimp.
As to why those instead of something like, say, tilapia, it's because shrimp are both wildly popular (meaning even a lot of people who don't like seafood will eat them),
And significantly cheaper than something like fresh salmon or tuna (in addition to being low in mercury).
You also have better portion control with shrimp since they come in individual pieces, meaning you can serve a skewer as a side or an appetizer,
Which is great news considering how well shrimp goes with steak. You can't do the same thing with, for example, salmon.
In all, it makes sense that shrimp is such a popular menu item in steakhouses -- and that it's so good when you order it.
That doesn't mean you can't make great shrimp at home, but professionals are professionals for a reason.