Why is Sankhya regarded as Āstika rather than Nāstika?

Although I knew the Sankhya school agreed with the authority of Veda, its believers and followers thought the original beginning only existed two entities, Purusha( psychical or mental spirit ) and Prakrit. Additionally, they don't believe in personal gods, including Ishvara. Essentially, Sankhya is considered an atheism of Indian philosophy, but even so, it's still classified as orthodox rather than heterodox. I'm a bit confused. Could anyone else elaborate on why it's like this?