Why is a shower stall not premitted in the same room as a high efficincy furnace

I recently had a new high efficiency furnace installed in my basement in the same room as there was an existing shower stall. The installers insisted on cutting and capping off the lines to the shower, saying it is illegal to have both in the same room. I wondered why they cannot be in the same room but did not get an answer from the furnace installers.