It was popular before zypper showed up. I guess smart rightfully died out, because it was even slower than yum. That is to say, if I have to wait for smart for on 45 minutes on a TM5800 CPU on a dist-upgrade for what "zypper dup" will do under a minute on the very same silicon, something is wrong. IIRC, smart was also a heuristics-based solver like most other implementations. It boasted on its then webpage to solve some cases that apt got wrong, and while that was true, I suppose it were these extended algorithms that eventually were able to drive the computation time to unbearable infinity.