Why important characters from Mortal Kombat: Legacy season one doesn't appear in season 2?

I don't there is an overarching reason, but some specific examples can be discussed. Kevin Tancharoen, the director of the second season, said in an interview:

On the absence of Michael Jai White (who played Jax) and Jeri Ryan (who played Sonya Blade) from season one:

"Scheduling wise Jeri was on a hit ABC show, so she couldn’t come and join us and Michael Jai White has BLACK DYNAMITE amongst other movies he’s doing. And beyond that, I didn’t want to have to ask them to do me any more favors...However, the creative decision to not have them in this season is because I want to save them for the feature film. So, that is still for sure, in my mind, where you will see them again."

To confirm this, Jeri Ryan posted on her Twitter account:

@Kitana_MK2013 Sorry, not doing Season 2. Still shooting season 3 of Body of Proof.

Michael Jai White posted something similar on his Twitter account:

@sweetre15 I'm coming back to do the Mortal Kombat movie but not the web series.

Darren Shahlavi (Kano) also used his Twitter account to comment:

“@Liz82002898: @Shahlavi @YoungerSubZero HNY x when can we expect you again in Mortal Kombat Legacy Kano..?”no Kano 4 llegacy2unfortunately

When pressed on why (and visible on the above link), he said:

@JamesWrightNI @liz82002898 @youngersubzero Kano Jax and Sonya origins were told in season 1

So that's hopefully given some reasons as to at least three of the characters - Kano, Sonya and Jax - not returning.

I'm aware that is only a subset of the characters, but it appears that a mixture of one-season-only origin stories, combined with the actors having other commitments, led to quite a few cast changes for Season 2.