*this is not an opinion just an objective explanation based on the information I have

The reason that churches aren't taxed, legally, is because the US constitution states, "Congress shall make no law respecting an establishment of religion" There are disagreements about whether taxing everyone including churches counts as a "law respecting an establishment of religion" but that is the current state of affairs.

*this is an opinion, though more of a speculative one

The reason, I believe, that the law mentioned in this post isn't enforced, is because if the did the supreme Court would likely through the case and the law out as a result, for being unconstitutional, as it is unarguably a "law respecting an establishment of religion."