The philosophy with Slackware is to be as simple in design and stable as possible. Here, a text-based interface is relatively simple and it's been extremely stable.
The strive for simplicity and stability is also another reason why Slackware typically only ships with tried-and-true programs--and a 2.4.x kernel by default. (though a recent 2.6 kernel as well as other, more cutting-edge programs are included in /testing).
Also, IMHO, Slack's installer is probably more semi-GUI than a true text-based installer. Is it anaconda or YaST? No. Is it simple, stable, and effective? Yes.
It looks like it's ncurses, but that's just a guess. I've always found Slack's installer quite easy to use and somewhat reminiscent of FreeBSD's.