I think there’s at least 3 factors at play here.

First, you’re probably living in a largely eurocentric bubble. You’re not seeing other mythologies because they’re not being marketed to you, and in some cases you may not even realize some of the ways that those mythologies and folklore and such are being presented to you because you just don’t know what to look for (for example, Dragon Ball, in the beginning, borrowed very heavily from the 16th century Chinese novel Journey to the West, which is a hugely important book in Asian literature, and I swear every couple of years there’s some new adaptation coming out, but it’s not nearly as well known to Western audiences) and translations can get a little wonky, if you watch a movie or read a book from a non-western culture, instead of naming specific deities or other mythological figures, the translator may figure that no one reading the translation is going to know who that is so they’ll translate it as something generic like “god” or “a great hero” instead of naming names.

Second, Western media is huge, and kind of overshadows a lot of other cultures. White Americans making movies in Hollywood are going to tend to pull from their own cultural backgrounds, and that often includes Greek, Norse, and Roman mythology.

Finally, a lot of it comes down to which mythologies we have actual written records of. The Norse, Greeks, and Romans all wrote about their gods to some extent, Slavic people, on the other hand, did not write until after they’d been converted to Christianity (the Cyrillic alphabet used in Russian and some other Slavic languages takes it’s name from Saint Cyril, who helped to christianize the Slavic peoples, and was developed by his followers,) so there’s no real first-hand accounts of their beliefs and practices, only second-hand accounts from other cultures who interacted with them and wrote down what they observed, and people recalling stories they’d heard about earlier times, and that comes with them inserting their own biases and interpretations and just plain getting things wrong. So if you wanted to write something about, for example, the Slavic gods Perun and Veles, you probably wouldn’t have as much decent source material to work from as if you wanted to write about the roughly equivalent Norse gods- Thor and Loki.