How long a space trip lasts is much more a factor than some think. More important than the energy it takes to get to the destination.
Here's one more issue.

It's a cumulative issue.

The longer the trip lasts:

-The more mass it needs to carry (food, water, living space, comfort, shelter), the more energy it needs.
While a short trip needs less comfort, etc...even if more energy is needed (relative to its mass) (for instance to the airless Moon), the cumulation of mass for a longer trip (to Mars with an atmophere) will demand more energy anyway.

-The higher the risks of exposing humans to dangerous radiations, longer weightlessness, boredom (total black sky outside), isolation from Earth, temper among crewmembers...

-The higher the risks of corrosion, leaks..

-The longer it will take to train humans and equipment..

-The less attention from the politicians, the media and the public...

But a short trip (around two weeks) has none of those issues or to a lesser degree....and results would come faster!

The Moon still has appeal for manned spaceflight