'Beasts of England' is sung very enthusiastically by the animals. Why? Why is it easy to remember for even the stupidest of them?

Near the end of the first chapter, when the animals sing
this song, Orwell writes the answer himself to your very question... at least in
part:

The
singing of this song threw the animals into the wildest excitement. Almost before Major
had reached the end, they had begun singing it for themselves. Even the
stupidest of them had already picked up the tune and a few
of the words, and as for the clever ones, such as the pigs and dogs, they
had the entire song by heart within a few minutes. And then, after a few preliminary
tries, the whole farm burst out into 'Beasts of England' in tremendous unison.
The cows lowed it, the dogs whined it, the sheep bleated it, the horses
whinnied it, the ducks quacked it. They were so delighted with the song
that they sang it right through five times in
succession, and might have continued singing it all night
if they had not been
interrupted.

In this passage,
we see that the "stupid" animals could pick it up because of the tune and some of the
words. To me, this means that familiar sounding 'Clementine' and 'La Cucaracha'
motivated the animals. The language of the song must have pleased them as well. No
matter how dumb an animal or person is, if you give them what they want to remember, it
is easier to remember.

Other reasons for the ease of
remembrance this passage shows include the facts that every animal could sing it in
their own way, and they repeated it 5 times.