She wants to run 10k at the Olympics because she is too late to be fit enough to compete at the Olympics in the marathon.
From RW:
"With the Marathon Trials on February 29 and the Track Trials in June, she’ll have more time to prepare and earn an Olympic standard in the 10,000 meters (31:25). But Jorgensen is still looking at the marathon long term."
Source:
https://www.runnersworld.com/runners-stories/a30913876/gwen-jorgensen-comeback-to-racing/