One of the reasons is security, older Android APIs let your apps do whatever they want. You probably could try to find an apk that would work on older devices, but google has minimal API requirements that involve permissions and data access, and if your app doesn’t support them it wont be allowed on the Play Store. So even if devs supported other devices, you wouldn’t find the app on the store.

Not to rag on you, but do consider security risks associated with using an unsupported OS. Take care.