Subject: Why is interplanetary travel limited to ballistic trajectory flight?

A trip to Mars takes 180 days (or so) to reach Mars' orbit. A window for the 
return will not open up for another 26 months, meaning that a round trip takes 
something like 3 years. How realistic (or not) is it to expect direct powered 
flight at speeds that are, say, an order of magnitude faster than orbital 
escape velocity? Reason: I am writing a "futuristic novel", not necessarily 
Sci-fi, that is set in mid-century (this one, that is). I am aiming for an 
authentic look and feel for the technology and the storyline.

Re: Why is interplanetary travel limited to ballistic trajectory flight?