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

No — you don't pay, so the premise is off. Heisho's text is "Players ignore [Deflect] while paying for spells and abilities choosing something here," and that blanks the Deflect instances for the entire payment of the spell, not just for the target sitting at Heisho.

Since Star-Crossed chooses your unit at Heisho, it qualifies as "choosing something here." Ignored abilities are treated as inactive for that game action (Rule 766), so the enemy unit's Deflect is inactive while you pay Star-Crossed's costs — even if that unit is outside any battlefield at its Base. The Deflect permanent doesn't need to be the thing you chose at Heisho (FAQ #11783).

You'd only pay Deflect if nothing Star-Crossed chooses was "something here" at Heisho — e.g. both targets are elsewhere.