Why did Aristotle reject Plato’s belief in separate transcendent Forms?

Aristotle rejected Plato’s theory of separate, transcendent Forms primarily because he believed the theory failed to explain the nature of reality and the process of change.1

His objections included the following points:

• Lack of Explanatory Power: Aristotle argued that positing a realm of immutable, everlasting Forms outside of space and time does not help explain how particular things come into existence or undergo change. He famously contended that the theory merely introduces new entities equal in number to those it seeks to explain, effectively doubling the problem rather than solving it.1
• Immanence vs. Transcendence: Unlike Plato, who viewed reality as existing in a realm remote from the senses, Aristotle maintained that forms are immanent—they exist only as constituents within particular things. In his view, a universal (such as the species horse) is not an independent entity but a common structure embodied in the matter of each individual horse.2345
• Individuation: Aristotle argued that what makes two things of the same kind distinct from one another is not their form, but their matter. Consequently, he shifted the focus of philosophy from a transcendent realm to the "here and now," grounding his metaphysics in the study of concrete, material substances.24