I mean, plenty of civilian characters had first names too (Akiyama Makuro, Nathaniel Zib, Daniela Capricorn, etc). It was probably to make them more relatable — showing that in this universe, despite being robots, they're also people like us.

The same could be said for other decisions made early in the theme. The heroes were given handwritten "signatures" and very human vulnerabilities, like Bulk's self-consciousness about his intelligence or Surge's doubt in his own heroic mettle, not to mention gags about the heroes doing product endorsements for Power Core or having their adventures depicted as TV shows and musicals. These, among other things, are part of why I think Hero Factory was a much smarter theme from the outset than many people give it credit for.

As for why the first names were dropped in later years, I'm guessing it's because the characters mostly referred to each other by their last names anyway, and because as the series got more and more characters the creators wanted to simplify the list of character and set names for fans to keep track of.

The Bionicle comparison doesn't quite work because the Bionicle theme's names were designed to make the characters and world seem as "exotic" as possible, rather than just a futuristic, robotic counterpart of our own western society.

I will say it kinda worked. I respected Bulk for making attempts to further educate himself.

I try not to concern myself too much with anything to do with hero factory