You would think they did not want to be put on the spot about the presidential succession and Trump's refusal to allow the transition to move forward.
Exactly.
Knowing the host would want them to
a) speak for President Trump
and/or
b) defend him
when there are so many other important matters they could be talking about.
These are legislators - and should be treated with proper respect. When it is easy to know they are not to be, avoidance is their right - and their wise option.