Why is the Piper PA-23 considered a bush-plane, while the Beech 95 Travel Air is not?
Without getting into the details of the two, there are going to be 3 factors:


Landing Gear suitable for rough fields.
Decent size cargo loading doors and cabin with cargo provisions.
Power to weight ratio suitable for short rough fields.


Neither of those airplanes are really purpose-built for bush operations, but to the extent they have those characteristics they could be said to be more or less suitable.

When the US Army was looking at the DeHavilland Canada Beaver, which WAS specifically designed for bush operations, being designed in response to an operator survey, there was a hue and cry from the US aviation lobby, prompting the Army to have a fly-off with the Beaver vs a motley assortment of "utility" aircraft put forward in haste by various manufacturers (including the Travel Air IIRC, and the Cessna 195).  The Beaver won without much difficulty, and the US Army went on to take nearly 2/3rds of Beaver production.