Trying to phrase this neutrally without going into detail, it's basically that she was pretty much The Only Person Doing Mando Stuff in the EU for several years starting with Hard Contact in 2004. When the 2008 Clone Wars cartoon came around and also wanted to do Mandalorian stuff, there were (inevitably) continuity issues and and Filoni's show won out over her novels/short stories. Her writing career was strong enough that Star Wars was only part of it, so she moved on to doing other things.

There were a lot of strong feelings from all different sides about it.

Traviss is very good for clone and Mando feels.