Local churches seem to be an exception, without any apparent rationale.
There are a variety of ways to make an appeal. You could use your monthly newsletter or the Sunday bulletin to inform people your church welcomes and encourages such gifts. You might send a letter with a brief, informative brochure, as St. Mary's, Scarborough, has done for several years. It is not enough to mention the tithe. A program should give people a way to reach it.