There seems to be a move away from Tithing to 10+% Giving. In other words, recognizing that Tithing - the Old Testament command to the Jews to give 10% of their salary to support the temple priests - is theologically indefensible, pastors are now using emotional manipulation to get their congregation to give more than 10% of their income. "If Jesus gave His all for you, shouldn't you give more than 10%?".
Short answer of why Christians do not have to Tithe.
Tithing was under the Old Covenant to support the priests of the temple. In the New Covenant, all believers are the priests - "priesthood of the believer" 1 Peter 2:9. Our bodies are the temple. We give (not tithe) to support full time evangelists and needy believers. Everyone else should have a secular profession to support themselves "each of you should work with his hands" 1 Thessalonians 4:11. Pastors, teachers and all other church staff should be paid for their work - but part-time, not full-time salaries.
Short answer of why Christians do not have to Tithe.
Tithing was under the Old Covenant to support the priests of the temple. In the New Covenant, all believers are the priests - "priesthood of the believer" 1 Peter 2:9. Our bodies are the temple. We give (not tithe) to support full time evangelists and needy believers. Everyone else should have a secular profession to support themselves "each of you should work with his hands" 1 Thessalonians 4:11. Pastors, teachers and all other church staff should be paid for their work - but part-time, not full-time salaries.