Why is there no names of any Mycenaean kings outside Greek mythology?

The surviving documents from Bronze Age Greece are products of the Mycenaean palace culture, ranging in date from ca. 1400 to 1200 BCE (plus one or two decades on either side).

All the documents that have survived, survived by chance thanks to being filed in a temporary storage area (not long-term archives) and being fired when the building was destroyed. And that’s what the texts are: administrative records of economic activity, not political or historical records. The New Pauly sums up their content as
instructions, inventories, deliveries and allocations of persons, animals, and goods, furthermore receivables and deficits of the accounting year in which the respective residence was destroyed by fire.
Plenty of names do appear in these documents — nearly 2000 names (the number may have passed 2000 by now) — and they are people who had economic interactions with the palatial centre: herders, craftspeople, landowners, military officers, priests and their gods. Not rulers.

Our only chance for knowing any rulers’ names would come from diplomatic correspondence with the Hittite king at Hattusa, and some correspondence does survive, but unfortunately not with any names.

Names in classical-era myths are just that: classical era (700 BCE and later), and mythical.
the term wanax (the Mycenaean Greek word for “king” or “ruler,” also attested later in Phrygian as vanak) is exceedingly rare in Linear B records, appearing roughly two dozen times out of the ~5000 surviving Linear B tablets. There are seemingly no references to queens at all, as the feminine equivalent wanassa was reserved for goddesses with at least one possible exception; see “The Wanassa and the Damokoro: A New Interpretation of a Linear B Text from Pylos” by Jorrit Kelder and Marco Poelwijk.
https://repositories.lib.utexas.edu/bitstream/handle/2152/63630/palaima_1995a.pdf
Some Mycenologists believe that the Ekhelawon (e-ke-ra2-wo) mentioned in tablets from Pylos should be identified as the wanax of Pylos based on his apparent wealth and importance, but this remains uncertain since he is never explicitly referred to as such. There’s a discussion of the relevant texts in Thomas Palaima’s article “The Nature of the Mycenaean Wanax.”