It was not so much him not being able to reinforce his forces but more the fact that even in a victory he had taken disproportionate losses. Traditionally casualties were incredibly lopsided between victor and loser because most of the killing happens when one side routs and the victors run them down and kill them. It's a lot easier to kill someone who's got their back to you trying to flee having dropped their weapons and shield out of panic, in contrast to someone who's facing you actively defending themselves and trying to stab you at the same time.

In Pyrrhus' victories, at Heraclea the battle had gone on for an extended period before the Romans finally broke. Asculum was a two day battle that he just barely won. The Roman army was fighting much longer and causing casualties the whole time. When they did finally break, Pyrrhus wasn't able to inflict the kind casualties during the rout compared to other victories against the Romans. Both Heraclea (7,000 Roman dead out of 45,000 engaged) and Asculum (6,000 Roman dead out of 40,000 engaged) saw him inflicting less than double of his losses. In contrast, the Roman defeat at Noreia against the Cimbri saw them lose 24,000 out of the their initial force of 30,000.