Why you still have to pay for deflect while starcrossing yout unit in heisho and opponent unit outside battlefield

No — in that spot you don't pay the Deflect at all. You'd only pay if the spell isn't choosing anything at Heisho.

Heisho, Shell of the World says "Players ignore [Deflect] while paying for spells and abilities choosing something here." Star-Crossed chooses your unit at Heisho, so the spell qualifies, and Nasus's Deflect is treated as inactive while you pay its costs (Rules 766–767). The ignore is not limited to the object sitting at Heisho — the FAQ on exactly this scenario (Star-Crossed your unit at Heisho, enemy Nasus at Base) rules no Deflect is paid, because the card that qualifies you and the card with Deflect don't have to be the same card (FAQ #11783).

The "outside the battlefield" part is why it feels wrong: Deflect only asks whether the spell chose the Deflect unit, not where the spell's other targets are (Rule 809.1.c). So if the spell isn't choosing something at Heisho — say both targets are elsewhere — then Nasus's [Deflect 2] applies and costs you [A][A] in any-domain power (FAQ #5599).