If the temple practices were stopped why did Paul take a vow that involved a sacrifice when James and all the elders told him to? See Acts 21:17-26.
The same reason you pay taxes to the government you live under. That was the only economy the Jews ever knew. God did not change their government. God left the building, literally.
Government changes because the people change. Not because God proclaims one has to live under a certain government, so deal with it.
You base 70AD on the rules set up in Deuteronomy, correct? The Covenant God made with Israel at mount Sinai, did establish that government relationship with God. But you also have to figure in, that they cast off that government for a kingdom with a king. The people changed and wanted a new government like the other kids on the block.
Yet the economy remained the same, and God still held up His yearly visit, as long as the Covenant was in good standing. God did not dismantle their economy. Had they not revolted against Rome, they could have kept that economy, as long as the people did not change and decide on something different. The people decided to revolt in a drastic fashion like when the US decided living under British authority was too much. It worked out great for the US, not so much for those in the first century under Rome.
Even during the 500 years between the Babylonian captivity and the Cross, they had their good days and bad days. What went on at the temple was centered around an economy. You may see that part of that economy was spiritual. However it was their way of life and their physical economy that defined their whole existence. Some of that economy even filtered into the church, but defined through the process given in the book of Hebrews. Obviously they still kept up the physical part, even knowing that the Cross changed the spiritual dynamic. But the Jewish revolt was as much as a violent civil conflict as much as it was a conflict against Rome. Obviously not every one wanted to revolt against Rome, especially those who made a living as a go between Rome and the Jews.
Paul pointed out he was an equal opportunist, for lack of a better term. Not sure why that is difficult to understand. 1 Corinthians 9:19-23
"For though I be free from all men, yet have I made myself servant unto all, that I might gain the more. And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law; To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law. To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some. And this I do for the gospel's sake, that I might be partaker thereof with you."
Paul made full use of his relationship with God, and even his Roman citizenship. That does not mean that God was still part of the OT economy after the Cross.