Why did Rockstar change the formula for expansions from GTA3 to GTA4?

[Slimebeast]

If they weren't expansions then why weren't they (Vice City and San Andreas) labeled as GTA 4 and GT 5?

Yes they're expansions, kind of.

• GTA III
   - GTA: Vice city
   - GTA: San Andreas
• GTA IV
   - GTA IV: The Lost and Damned *
   - GTA: The Ballad of Gay Tony

You get the picture?

(*Yes, The Lost and Damned is preceded by the number GTA IV, while Gay Tony is not.)