There's no America/Salt_Lake_City you're recommended to use America/Boise instead. The people in Salt Lake City are about as far away from Boise as you can get and Salt Lake City is more easily recognized as a landmark then Boise. The process of choosing which cities should be landmark cities comes across as faulty and uninformed.
Boise has its own entry because in 1974, the Emergency Daylight Time Act shifted when DST began in Southern Idaho and eastern Oregon. Boise is the largest city in the region.
Technically, if you're in Salt Lake City, you should be using America/Denver, not Boise because of this, otherwise if you say, opened a calendar from 1974, everything will be off by an hour.
If Utah made DST there begin a day earlier this year, Salt Lake City would probably get an entry too.
The issue with this thinking is that all existing datimes referring to that region will continue using the old timezone because the new one didn't exist yet.
You can't really escape the fact Europe/ is going to be after Africa/ and America/ and Antarctica/ and Asia/ and Australia/ and Canada/ and Etc/
It’s common for major cities to be located on rivers that are state boundaries, the area around the city uses the same timezone, and one of the states has a timezone boundary in the middle. Indiana has many tz database entries because of this kind of thing.
There are other fun cases like the Navajo Nation in Arizona.
I live in South Dakota, which is one of them—the Mountain/Central timezone boundary within the state follows the Missouri river. (Locals refer to "East River" and "West River" to refer to the two halves of the state. The capital, Pierre, is technically East River, but is right on the banks, almost dead-center.)
[0]: https://simple.wikipedia.org/wiki/List_of_U.S._states_and_te...
For instance, both Sydney and Melbourne both have entries because Victoria and New South Wales have started/ended daylight savings time in various years since 1970 on different dates like in 1990 when in Victoria it ended on March 18 while in New South Wales it ended on March 4.
It's also why Broken Hill has an entry despite being in New South Wales, they're in their own timezone.
I think it's unlikely they're chosen as landmark cities. More likely, timezones were uniform, then some government likely did their own thing in their jurisdiction. The change was then represented as a new timezone named after the place where the change is centered. IOW, the names have more to do with some random divergence that happened at some point in history, rather than what landmarks are the most recognized for today's timezones. Re: Boise & Salt Lake City, maybe Boise was the first to do their own thing while Salt Lake City had a different timezone. Maybe Salt Lake City later decided to adjust their timezone to fit Boise's to facilitate commerce between their states.