The HL2 engine didn't exist when ESF laid it's groundwork, so they're stuck with it. I'm surprised however that HL coding wouldn't mesh with the Source Engine. I'd have figured Valve would make converting from HL to HL2 relatively painless. If you want to see DBZ in HL2, check Dragonball: Source. Be warned the mod is very much an early alpha and is only barely playable.
The only mod I know of who's willing enough to switch engines to improve the overall game is ZEQ2, who have actually scrapped their whole Quake3 designs in favor of a stand-alone engine called Zios.