Short answer: Because 2^10 (1024) is very close to 1000, and it’s more convenient and accurate to measure things in powers of 2 when dealing with computers.
You can either have a 8KiB cache or a 7.8125KB cache, for example.
At one point it was even worse because they were BOTH kilobytes and both written KB, and it was ambiguous which one you were talking about.
Grats Saba! Now on to 2M!