I think the main issue is the majority of the phone booth parts are made of clear plastic, and to produce all the necessary parts without distortion is not a low budget thing. I feel safe assuming @SteveG and the folks at Round2 thoroughly investigated the most cost effective way to get it done, and while the final choice doesn't appeal to me, I can respect the decision.
The '63 Nova Chevy II wagon had a telephone theme going on and related parts included (phone, decals), so while that could have been a potential kit in which to include a phone booth (especially in light of the trailer inclusion), I'm not sure how well that, sold at a premium price, would've been received by the buying public. A cab forward Dodge A-100 pickup might fare better, at least in terms of a Ma Bell type service vehicle. Hopefully the thin areas of the bed floor were addressed, too.