I got my Looney Tunes Replacement disc today. For those who don't know what I'm talking about: disc 4 of the Looney Tunes Golden Collection Volume 2 had a transfer glitch that caused several cartoons to receive "interlaced" transfers, which makes them choppy and shuddery on many TVs and computer screens. They corrected it and you can get a replacement copy by calling 1-800-553-6937. Anyway, I now can watch "I Love To Singa" without it looking like crap on my TV, coincidentally just in time for a DVD set of The Jazz Singer that also includes "I Love To Singa" (since it's a cartoon takeoff on that movie).
I think this is one of those cartoons that's been elevated to historical classic status by popular opinion. The critics and animation historians rarely mentioned "I Love to Singa" as a seminal Warner Brothers cartoon, but anyone who ever watched the pre-1948 cartoons on local stations recalled "Singa" as one of the best cartoons they'd ever seen. I remember seeing it in the middle of one of those dispiriting blocks of pre-1940 Merrie Melodies, the type that gave pre-1940 cartoons a bad name (after twenty minutes of round, bouncy characters singing songs you'd never heard of, you'd long for a Daffy/Speedy cartoon). I was hooked; this was clearly an "early" cartoon yet it grabbed my attention, made me laugh and sing along, and just felt totally fresh and modern in a way that most '30s cartoons did not.
If you go through the archives of alt.animation.warner-bros you'll find that seemingly every other week there was someone writing in to ask "what's the name of that cartoon with the owl who sings 'I Love To Singa About the Moon-a...'", and of course the first episode of South Park refers to this cartoon and the song. It is, really, the first Warner Brothers cartoon that needs no historical excuses: it's not great for an early WB cartoon, or great because it broke new ground, it's just great because it never fails to be a huge hit with audiences.
As for why this cartoon grabs so many people, I think it's a cartoon that combines strong storytelling (even if the story is largely cribbed from The Jazz Singer) with a modern satirical style. It's not a gag cartoon, like most non-Disney cartoons in the '30s, and it's not a purely musical cartoon, like a lot of the Merrie Melodies; it's a story/character cartoon like "The Three Little Pigs," but with a Tex Avery edge to the humor.