Actually, the evidence indicates that
Happy Birthday is not under copyright.
Edit: Correction to above: I meant to write "should not be under copyright". I should not have written that I disputed that it has a copyright, but instead that copyright should never have been assigned.
The main reason that "Happy Birthday" still gets royalties is that it's cheaper to pay the royalties than contest it.
The
melody is from the 1893 song "Good Morning To You" (now in public domain).
The
lyric was grafted from a pre-existing version of "Happy Birthday". It appears that grafting the two
may have been done by the author's students.
In fact, "Happy Birthday To You" was published (not by the authors) in 1912 with instructions to sing the song to the tune of "Good Morning To You"
with these alternate lyrics.
Then in 1935, a company claimed "Happy Birthday To You" was
their own work - over 20 years after it had been published in that form.
They'd been profiting from it ever since.