Two reasons. I love and trust 4wd. Looking for a van with 4wd set my imaginary purse screaming.
The other reason was cost. To get a slide in for a short bed was averaging $16,000. And there was the question of weight. The truck could just handle the weight, but I would be severely restricted on what I could carry. There are very few slide-ins for this short bed.
So, then the option of a trailer came about. The truck is rated to tow 6,000 lbs or more (unlike the 1,250 lbs it could carry on its bed). Hmm. Once again the price of a tiny trailer was boggling to my financial strings. A new one could cost $17,000. And I remember driving horse trailers and cattle trailers.  Not too much fun navigating tricky places. And I wanted tricky places.
I must confess that I have totally fallen in love with  tiny vintage Shastas. I love the ice box, the permanent bed, the oven(!), and their size. Once again it comes to price and not a fan of towing.
So, to the drawing board. I have lived in the back of a truck. I could do it again. And for sure it would be cheaper than buying a trailer or a slide-in.