This adds a couple more slightly speculative reasons to L.Schoon's list, which I commend.
Solubility: In ice cream, the pistachios are blended in with water and fats. When you eat them whole they're not. Any fat-soluble flavour compounds will be much more available in the ice cream than when chewed (and mixed with saliva, i.e. water). Some flavour compounds are definitely much more fat-soluble than water-soluble, such as capsaicin. Actual sugar and salt are of course very soluble in water.
Mouthfeel: this has a complex interaction with taste, but is all part of the experience of eating - think of the difference between buttered toast, dry toast, toast with jam (with/without butter).
These actually overlap in practice, and there's a whole field of scientific literature on the subject; one relevant paper is Effect of Fat Level on the Perception of Five Flavor Chemicals in Ice Cream With or Without Fat Mimetics by Using a Descriptive Test. The main conclusion for this question is that some flavour compounds were detected more strongly in full-fat ice cream, while others were detected more strongly in reduced fat recipes.