Why the time kept at the ISS is UTC? And Einstein's time?

The International Space Station (ISS) uses Coordinated Universal Time
(UTC), also known as Greenwich Mean Time (GMT), as its standard time.
UTC is the scientific standard of timekeeping for the world and is based
on atomic clocks.
The ISS is a partnership between five space agencies from 15 countries.
The station is continuously operated 24 hours a day, 7 days a week, 365
days a year. Crews from the U.S., Russia, Japan, Canada, and Europe live
and work on the ISS, which orbits Earth every 90 minutes.
The shuttles also had UTC clocks so that the astronauts could easily
figure out what the "official" time aboard ISS was.
It's a compromise for the Americans and Russians. The crew day begins at
about two in the morning in Houston and ends at about eleven at night in
Moscow.
So, for ISS, t = t'. Einstein's SR time is not even considered, even
when there are several atomic clocks onboard.
More yet, any space vehicle used to transport astro/cosmonauts keeps
time using UTC/GMT.
Reality dictates that the world is used to t = t'.
EXAMPLE:
The Cassini space probe used:
- Spacecraft Event Time (SCET), also known as Orbiter UTC, to keep time.
SCET is the time at which an event occurs on the spacecraft.
- Coordinated Universal Time (UTC): The worldwide scientific standard
for timekeeping, based on atomic clocks.
-One-Way Light Time (OWLT): The time it takes for a signal to travel
from the spacecraft to Earth. From Saturn, OWLT can range from about 1
hour and 14 minutes to 1 hour and 24 minutes.
- Earth Received Time (ERT): The time the spacecraft signal is received
at mission control on Earth. ERT = SCET + OWLT.