You'd definately have had to have lived through Dunsparce in order to understand why it was worse than Pachirisu. For starters, like it was said earlier, Dunsparce could switch with a benched Pokemon once it used Strike and Run, which gives just a huge advantage compared to Pachirisu, who has to wait out a turn before retreating. Pachirisu I'll admit does have a stronger attack via Smash Shot, but it does leave you open for an easy OHKO; considering the only Tools next format (unless some are coming out in future sets or being reprinted) are Amulet Coin and Leftovers (neither of which are groundbreaking or easily accessable next format since Castaway is leaving as well), Smash Shot's usefulness is limited to the rest of this format basically (which even then is limited usually to Cessation Crystal, which only a handful of decks play anymore anyways).
And of course, there's one more reason why Dunsparce is superior: he's a Common, unlike the Holo Rare Pachirisu, which limits access to Pachirisu considerably when compared to the easily obtainable Dunsparce back then.