The 390 years punishment were meant for Judah so why do we subtract 701 from 2730 when 701 wasn't a siege against Judah?

The 390 is a punishment against the House of Israel, not the House of Judah.

Ezekiel 4:5 (caps emphasis only)
For I assign to you a number of days, 390 days, equal to the number of the years of their punishment. So long shall you bear the punishment of the HOUSE OF ISRAEL.

The 701 aborted siege of Jerusalem was an aborted siege against the House of Judah, as it was the capital of Judah (Southern Kingdom) at the time.

Maybe your question is why the 390 for the House of Israel at the 701 siege when the 701 siege was against the House of Judah?

We do that because the prophecy demands it.

Ezekiel 4:3 (caps emphasis only)
And you, take an iron griddle, and place it as an iron wall between you and THE CITY; and set your face toward it, and let it be in a STATE OF SIEGE, and press the siege against it. THIS IS A SIGN FOR THE HOUSE OF ISRAEL.

Thank you. You have perfectly answered my question. I appreciate the response.