Sometimes it has to be a collage as no one thing is highly probable, but combined is.

That said, the likeliest issue to overcome is the Van Allen belt.

NASA says they found the thinnest part and the astronauts only spent a little time going through it. However the radiation is intense and the astronauts did not seem to be affected at all.

It is discouraging as an inability to pass through the Van Allen belt means there will be no way to get to Mars except through robotics. Perhaps humans could be incubated there?

Let’s see what SpaceX can do and why not go (back) to the moon before Mars.