There are many scripts from ancient civilisations ( for example Harappan script, linear A) that have not been deciphered yet even though we have advanced computers which can perform decryption easily.
It's not really a matter of computing power. More can always be said on the matter, so anyone with insight is welcome to contribute! In the meantime, since you mention Linear A, u/UndercoverClassicist has examined just why it's so difficult, also offering for comparison the matter of Linear B.