Why are Arabidopsis plant seeds being sent to the moon and not other seeds?
Without speculating further about specific decision making processes, Arabidopsis is the standard model organism in plant biology, similar to the mouse for mammals, the fruit fly for insects, C. elegans for nematodes, etc.

It's the first plant to have its genome sequenced, it's commonly used in undergraduate laboratory experiments in part because it grows very fast, etc.

If you were going to choose a plant arbitrarily for a research project, it might as well be Arabidopsis, I'd say it would be the default against which other options would be considered.