Green flame blade is a great horde killing spell while still feeling cool. IMO everyone picks booming blade because it's more useful against single targets, which is more fun against a larger range of enemies, from bosses to your equals, plus thunder is rarely resisted compared to fire.

Some people implement minion rules where overflowing damage from killing a weak enemy flows on to the adjacent enemy, which of course is simplified and incorporated into green flame blade. One of the hardest things to capture in the standard D&D rules is that in fantasy, the warrior (Aragorn, Holga, Achilles) typically cuts down hundreds of mooks while the mage battles the giant powerful monster who cannot be defeated by a sword (Gandalf Vs Balrog). In D&D, either it's totally inversed or the mage is better at both, largely because spells like fireball suit both situations better.

Green flame blade is a very easy option to balance this scale, albeit via magic.