Words have a ''sound''-an "a" sounds different from an ''o''. I'm fairly sure several big hits would have been lesser hits with different lyrics even if they have the same melody without even taking the content/meaning into consideration even if it's hard to prove (and no, I'm not counting songs in different languages that gets translated).

Also, there's a rhythmic quality to words that go beyond the ''sound''; certain melodies will sound rhythmically awkward with certain words; the singer will have to ''bend'' the words to fit the melody rhythmically which can work but it can also sound shit and then there's also the way a singer enunciate a word vs the way another singer do it.

TONS of factors basically and I'm not sure they are major but they are definitely not irrelevant...