Because T1 holdings — shares you bought earlier that haven't yet settled into your demat account — aren't in your account to deliver yet. When you sell them, that sale settles on its own T+1 cycle, so the proceeds are only realised at pay-out the next working day, not the same day. This is different from selling settled holdings, where 100% of the proceeds are usable for new trades immediately — see When do my funds settle after I sell Delivery shares?
Why can settled holdings be used at once, but not T1?
When you sell shares that are already in your demat, those shares are given to the exchange as an early pay-in — the depository blocks that quantity in your demat for delivery. Under exchange margin rules, once that early pay-in via the Block mechanism is accepted and the sale value is credited to your ledger, that value becomes available as funds/margin for other positions the same day. (NSE circular NSE/INSP/57112.)
T1 holdings aren't in your demat yet, so there's nothing to block and no early pay-in can be given. The sale simply follows the normal settlement calendar, and the credit frees up only when it settles at pay-out.
When does the credit become available?
At the sale's pay-out — the next working day (T+1). Selling shares before they've been delivered to you is the "buy today, sell tomorrow" case, including its dependency on the incoming delivery — see What happens if I sell shares before they are delivered?