If Turkey doesn't want a rat line into their country. We've seen this kind of thing play out numerous times over the years. Sharing a border with militants is never a good idea for the stability of your country.
Erdogan's comments are pathetic. The lecturing to the US on how airstrikes are not enough while his tanks watch from the hills just a few miles away is, incredibly sad and yet another fine example of a Muslim country not doing its share, even when indirectly threatened by militants of their own religion. And that indirect level of threat changes to direct, before you know it.
It's just a terrible policy from a guy who wants to punish Kurds (and kill Kurdish protesters) while he doesn't have the stones to commit his people to help take and hold a border town. I mean its not like there is an election to protect. Get off your ass and do what you promised.