Perils of party members choosing their leader
The trouble with giving people the vote is that they don't always vote the "right way". Democracy is rarely neat. Labour TDs were determined that the members should not choose the next leader.
Many senior politicians, across all parties, will tell you that it is important not to allow their activists have a huge say in framing the party's political direction. Just watch the choreography and stage management which goes into setting up the average political party conference. Keep the salt cellar close when you hear party principals talk about how their members chart the party's direction.
Too much "democracy" could be fatal to a major party's ability to woo voters and win power. The core argument is that the type of person who lives and dies for his or her party may not often be the best person to gauge the fickle public mood. Those party zealots just might want to foist their own doctrinaire views on people with a completely different take on the world.