Speed. Key generation (and most other operations) in ECC is order of magnitude faster than in RSA, DSA or ElGamal. Last time I checked it took End-To-End's Javascript library seconds to generate a key pair in RSA, but only milliseconds in ECC.
Key gen time probably doesn't matter too much in the particular case of pgp, but key size is a big deal. Keys that you might conceivably type by hand. Keys, not fingerprints, that can fit in tweets. Or tattoos. :)
Heh. No, I'm not trying to encourage that, but I think it's useful as rough measure of practical size for exchanging data. Like "Olympic swimming pools" is a popular measure.
