Because it's unfair towards the base defenders. There is no mechanic in game which would allow the defenders to spot the crouching infantry, and on maps with clutter and/or tunnels, the defending team may be AWARE of a Tanya in their base, and yet be completely unable to find her. This threat potential can cost the match. Think Tanya+Spy sneaking through the tunnels on KoTG, avoiding the mines.
Allied Minelayer is a huge time sink, which requires multiple repeated trips to refill. At the same time, the minelayer is unarmed and vulnerable to all forms of damage, which costs in repairs and repurchases. In return, 1 soviet player could buy a mammoth tank + engie and effectively clear the whole map in one go. It's completely disproportionate in terms of time and risk invested. Now, the soviet engie can still do it, but he needs a supply truck instead of a tank, which gives the minelayer a form of counter play.