I go out of my way to play MM at the location by me 'cuz I don't ever see owning one in my future. Maybe guys like me playing specific known premium titles?

No, MM has *always* made a bunch of money on route ever since it first came out, and it's earnings basically never dropped off. It's why even 10 years ago, you couldn't get one for less than NIB price - because they earned better than just about anything else.

The other titles really depend on a lot of factors. I have heard that there are a couple BBHs around here that do amazingly, but I know another op about 90 miles away that couldn't ditch his BBHs quick enough because they didn't earn at all. I have heard that POTC and SM are consistently good earners. There is a location that has a DM still on route. That op thinks the novelty of the handles still gets people to play it.

Look around your town and see what it still out. If it is more than a few years old, chances are that those are the games that are proven to make a lot of money for the op in your area.

I remember reading that South Park was extremely popular on route when it first came out (must have been the toilet on the playfield?)

Road Show does well due to the buy-ins to get across the country

Ironman because of the fun and short game times

Because it's not easy to know what the attractive qualities are and invent something similar.
You also can't copy them to another game - the same rulesheet on another playfield layout (with more difficult/easy shots) may not work.

What makes a game excellent is the combination of theme, graphics, sounds, flow and difficulty/easy of the shots, rulesheets (easy and difficult goals), variation (not always the same sounds, ..) and if you have it all you get a great pin.
To make it earn well on location it needs to be attractive enough to get people to play it. Make them have a great playing experience from the first shots, have some easy goals so they know what to do, but still have them not reach certain things so they want to play it again and try to achieve the other goals.

It's the same as saying why can't you make up a great joke by listening to a lot of other great jokes and copy these ? Or make a new song by taking the best things out of existing hit songs ?