My thoughts: I suspect they just used what they could work with, it didn't need to have bush wheels which generally are used to operate from very rough surfaces. It was for STOL research only. These wheels and brakes are ubiquitous, so easy to find and replace. I guess the forces on the struts are more balanced using this design as well.
And these days, bush wheels are expensive! Not really something you want unless you have plenty of money and thinks it looks cool, or really need them for rough terrain ops.