Some of them are. I think a lot of it has to do with that was the character they/most of us put the most work into starting out. Using skill books, grinding for gear, getting to 6 star etc, and masteries.

Also, most of them are universal meaning there great in many modes, Like Athel and Kael are great for CB, Elhain is pretty good in arena. They are great in many modes so makes them better than a niche unit only great in a single mode.

Rare Champions only need rare Tomes.

If you compare Athel to Hellgazer(Demon, Epic) Hellgazer ist "a little Bit better".

But to max Hellgazer i'd need Epic Tomes, for Athel i just need Athels and rare Tomes.

As i could use both for spider, and Athel got the faster Cooldown, due to Tomes, i equipped Athel, and Hellgazer ist still nakid.

..As i wont bring nakid Demongirls Info the Arena, i bring Athel.

Ist's the same Story for the Other starter Heroes.. It only comes down to Tomes..