Why did Quine accept sets as part of his ontology despite his nominalist leanings?

Although W.V.O. Quine began his career with strong nominalist leanings—famously rejecting abstract entities like classes and properties alongside Nelson Goodman—he eventually accepted sets into his ontology due to the indispensability argument.123

Quine came to believe that philosophy should be an extension of science, and he maintained that we are ontologically committed to whatever entities are required for our best scientific theories to be true. Because modern scientific and mathematical discourse relies heavily on set theory, Quine concluded that sets are indispensable to the structure of scientific knowledge.412

While he preferred a "desert landscape" and sought to minimize his ontological commitments, he ultimately accepted sets as the only abstract entities necessary to maintain the integrity of scientific discourse. He rejected other abstracta, such as meanings and propositions, as scientifically useless, but he could not justify discarding the sets required by the mathematics that underpins our understanding of the physical world.412