why did they not just buy the Marenco
Because the development costs would preclude Bell from offering the Short Light Single (God, what a name) in the sub-1-mil bracket they are currently aiming for.
Me on the other hand, I agree with you 100%. I would go for something like the Marenco with the view to creating an ultra-economic modern aircraft to run head-to-head with the Colibri. If I were Bell I would then package this with a low fixed rate of interest payment plan so that you only have to front 50% of the purchase price.