Why don't Christadelphians pay tithes?
The giving of tithes was a requirement under the Law given through Moses as part of the Old Covenant which God made with Israel at Sinai. The tithes were for the upkeep of the Levites who received no inheritance in terms of agricultural land. Every third year they were made available also to the poor.
Israel failed to keep the Covenant made at Sinai and God, through Jeremiah, promised a New Covenant which would be different to the Old Covenant. Jeremiah 31:31-34
Jesus fulfilled the Law given through Moses and taking it out of the way instituted a New Covenant. Hebrews 8:8-13
Jesus is the mediator of the New Covenant. Hebrews 12:24
When we believe the Gospel and are baptised into Jesus Christ we come under the New Covenant. We are required to obey the commandments of Christ, not the Law of Moses. Many Jews who were baptised into Christ wanted Gentile believers to be circumcised and keep the Law of Moses. A meeting was convened at Jerusalem over the matter. Acts 15:5-31
Tithing is not a requirement under the New Covenant. Under the New Covenant we are to give what we can afford. 1 Corinthians 16:1-7
A tithe of income can be viewed as a guide for contributions, but it is not a rule under the New Covenant. What Christadelphians give to a collection is anonymous and entirely a matter for the individual. Christadelphians do not have a paid ministry to support.
I hope you find this helpful,