The answer to OP's question is contingent on whether or not one means
300 the graphic novel and film, or the overall legend. As I began to write this response I realized the question was framed "300 of Sparta", rather than just "300". Responses to the OP thus far have probably pretty accurately summed up why portions of the far right enjoy the legend/myth/historical occurrence overall. Below is why they would enjoy the movie.
The movie 300 I will say has extremely racist undertones. And most of the time it isn't even undertones.
Indeed.
300 makes very explicit that it pits free, rational, masculine, and moral Europeans against despotic, mystic, effeminate, and degenerate Asians. Moreover, the movie (I cannot vouch for the graphic novel) takes no prisoners in granting the Persians access to weaponry and beasts we would expect out of India, Japan, and Africa--thereby at once creating a potentially interesting fantasy/mythical scenario while also bald-facedly making it "Europe versus the entire rest of the eastern hemisphere". While
Lord of the Rings has been accused of a similar dynamic (oliphants, scimitars, dark-skinned men from the south and east), Tolkien scholars can rebuff these claims in a way that I do not believe the makers of
300 are able to, even were we to grant that the movie may take place through the lens of "myth" (and I think the key distinctions here are that Tolkien was deeply respectful of Middle Eastern cultures and myth and incorporated them into his work, and that LotR at its essence did not revolve around cultural geopolitics;
300, on the other hand, has as its central point that confrontation of Europe and Asia). In addition to all this over-thinking,
300 is at its base a glorification of masculinity, force, and self-sacrifice.