Why do i have to pay for state and federal separate ?

[AkinaLO]

It's because state and federal are two separate entities. Your state has its own timeframe and timelines. 

Here is an article that may help a little. 

Do State and Federal refunds come in separately?

[PenPal]

Because they are 2 separate entities and one likely cannot accept the other's payment.  It would be comparable to paying your mortgage payment to your utility company.  The two don't "talk" to each other so you would need to pay each separately to ensure each receives your payment properly.