Simply because she is a hamster and it is easier for her to get up using the bars! Unfortunately rodents don't see the world as we do so she will grasp to a place using the quickest route! I have two gerbils in a gerbilarium (massively deep cistern so they can burrow with the wire cage segment on top) so there is a metal ladder which they can climb- to a human it seems easier but the gerbils seem to be to think it easier to scramble up the bars- from looking at the picture of your cage it looks like she would find it might easier to grip onto the bar!
If you are still worried take a look at the cage mine have-
if it's a dwarf hamster, that might be the problem
dwarfs have problems climing vertical tubes
maybe shes scared of the tubes try to push her head contained by the tube at the bottom and cover the way she went in and she will hav no choice but to cilmb the tube do that a couple time and she will see the tubes parsimonious no harm or maybe she think the bar are a better way to get up there
is it a dwarf? If its a dwarf, then its toooooooo small to go up the tubes and will slide back down. If its a regular big hamster, next maybe it might get stuck in the tube or its sluggish. My hamster used to go down and up the tubes and it got SO tired because it was tubby. XD
Well You got to wait until it trusts you it might expect its a trap or something get closer to it until it trust you and knows the home very well