Why a decrease in current asset is added to the cash flow statement?

There are a few reasons that vary based on the current asset you're referring to. If its a prepaid expense that's been decreased you've generally increased an expense. Like if you have prepaid insurance it may be amortized to expense over the year. So this expense flows into cash flows through the net income amount. But you haven't paid cash for this expense it was merely reducing prepaid expense from the prior year. So it gets added to cash flows.

If its account receivable that's being reduced it means in general you've received cash from your customer. But that amount is not included in net income as it was probably income and a receivable the year before. So you have to add it to cash.