Just very curious. There are three seats in the Russian Soyuz spacecraft, and sometimes just one astronaut of the three is a Russian and the other two are either one American and someone from yet another country, or another Russian and one American. Of course one reason is that since the end of the Space Shuttle flights there was a contract that Russia would send one American in their spacecraft to the ISS. The contract was to get expired in 2019 but obviously was prolonged (probably because the Dragon spacecraft once was planned to launch with crew already in 2014). However Russia already sent non-Russians long before 2011 to the ISS. The last Soyuz mission with only Russians was Soyuz TM-30, the last manned flight to Mir, in the year 2000. Why didn't Roscosmos launch any spacecraft with only Russians on board from then on?
As of 24 April 2020, the Russians & Chinese are the only ones capable of sending people into orbit. The Chinese crewed space program is still in its early stages & only sends Chinese people into space.
People from other countries would like to go to space & the International Space Station is funded by a number of countries, including Russia. The only way to get people from those countries to the ISS is via Russian launch systems. So far the Russians don't mind receiving money for providing a taxi service to & from the ISS & this is why there are non-Russians using Soyuz.
Also being International, it would not be considered good form if the crew of the ISS was from just one country, a balance needs to be maintained.
This is somewhat adding on to @geoffc's answer
The ISS is a joint project between NASA, Roscosmos, and other agencies.
As geoffc mentioned, both NASA and Roscosmos want to always have at least one Russian cosmonaut and one Astronaut from the US on board at all times.
The problem with the space shuttle was that it could only stay docked to the ISS for 12 days (it was initially 6-8 days because its fuel cells had limited capacity, and they were able to take that up to 12 days by channeling power from the ISS using Shuttle Power Transfer System (SSPTS), though the longest flight lasted 17.5 days on STS 80, which was not a mission to the ISS), while the Soyuz can stay docked to the ISS for more than 6 months.
This means that if Soyuz capsules launched with only Russian cosmonauts on board, the ISS might not have had any American astronauts on board until the next shuttle came by, and then again 15 days later when the shuttle left again with all the American astronauts.
Of course to solve this the shuttle would swap some astronauts to keep a rotating crew, and the shuttle would leave the ISS with different astronauts than it came with, but this wasn't always the case, and after the Columbia disaster the shuttle would rotate only one astronaut.
The Soyuz was still somewhat necessary to keep a permanent crew on board the ISS.
The Soyuz vehicle requires a custom seat liner to be make the impact of landing safer.
Soyuz capsules return to Earth via a parachute to a land based landing, uses solid rockets that fire in the last few seconds to dump the last of the velocity.
This is not the most comfortable of all possible landing methodologies and the custom seat liners are designed to make takeoff and landing more comfortable and safer.
Thus if you intend to launch or land in a Soyuz you need the seat liner.
During the Shuttle era, if an astronaut/cosmonaut launched or landed across vehicles, they have to bring the liner with them. (Mostly this meant Shuttle launched people brought the liner for later use in landing in the Soyuz).
In terms of crew, the minimum crew is usually 3 (occasionally was 2 during problematic times) and the US and Russia want to have a crew member on board at all times.
The maximum crew was 6, two Soyuz full loads, until the US gets Commercial crew launching when it will be 7 (3 on Soyuz, 4 on US vehicles). Thus any particular crew of a Soyuz will consist of at least on American, one Russian, and then who ever makes sense to fill the spot.
If they launched an all US Soyuz or all Russian Soyuz, when crew changes occur you would have a circumstance with only one nationality represented until the next crew launched.
This could be resolved by launching a third crew, before the other crew departs but has not really been done on ISS much, though it was done more on Mir. (I quibble since occasionally they will launch a Soyuz with a crew for a week or so, who then return in an old Soyuz, for long duration missions that exceed the in-space lifespan of a Soyuz vehicle).