I can repackage and rename whatever I want. That doesn't make it famous by default
So you want an answer to the other question too.
The answer is still nationalism and, to a lesser degree, anit-japanese sentiment.
Some time after the Japanese were finally forced to leave Korea at the end of WWII, Gen. Choi <cough> "unified" the kwoons under nationalistic sentiment. This included getting their story straight that TKD, as he dubbed it, is now, and always has been Korean, despite everyone knowing darn well it was repackaged Shotokan. You can tell by looking at the Hyung which was taught in the '50s. After he "unified" most of the kwoons (TSD being one of the hold-outs), the Korean government, again, in a fit of nationalistic pride, decided to promote TKD world wide, even going so far as to pay TKD instructors to live abroad and teach. But westerners were already quite familiar with Shotokan and easily looked at it and saw that TKD was just rebranded Shotokan karate.
So, mostly Nationalism.
But if you want to go into general psychology, by the time TKD, and other martial arts started showing up in the west, various Japanese karate systems had already been established, notably Shotokan, and humans naturally relate the new to what they already know. That's why Japanese Karate was sometimes referred to as "the Japanese system of boxing" and why Savate is still sometimes called "French Karate" despite the evidence that Savate and western Boxing influenced the creation of Shotokan instead of the other way around.
This is an effect that publishers of books took advantage of.
Peace favor your sword,
Kirk