Many do not participate because it must be done by hand for every client and that costs money. There are some hundreds of discount drippers, but most are very small companies. There is also the issue that the broker needs to go through the transfer agent as DAR stated. The transfer agent does not issue them the same day. Sometimes a day or two. Some brokers are crediting immediately to your account using company numbers or their own calculation and the broker waits for the transfer agent to issue the shares in a block.