There's a number of things to consider here:

1) I don't have the stats handy to back this up, but I recall seeing several mummy candidates with excellent chances give a wrong item, so that's dragging the average down a bit. Whether there's a correlation between 'the kind of player that can get a good chance to succeed' and 'the kind of player likely to give a wrong item' in a more general sense, I couldn't say.

2) I've seen at least a couple people power a necromancer to 47 and take a crack at mummy. These are characters that didn't have very good mummy success chances, but they're also characters that would have a roughly 0% chance to even get a lichquest. That also drags the average down.

3) You might be surprised to see the success chances for some of the surviving liches. I don't think I've ever seen a higher than 75% liching chance, and probably a number of liches you'd remember were closer to 50% or lower but beat the odds. I suspect nearly all of those characters would have had equal or higher mummy chances.

4) Mummying does have the advantage of being deterministic, in the sense that once you know how to do it, you can be guaranteed a quest. Depending on immortal availability, liching can be more hit or miss.

5) Finally, mummies have seen a number of small improvements over the last few years that, together, round them out pretty nicely. I no longer see them as strictly the poor man's lich; depending on your play style they may actually be stronger than a lich. A mummy is pretty much always stronger straight out of the gate or recently after a death. Liches only really shine relative to mummies for players capable of going a long, long time without dying to anything, regardless of whether or not they're looted. If you die enough to lose more than a point of con getting to mummy/lich, probably you should go for mummy. (I'm tempted to say any con.)

Basically, if you would be willing to play A-P if any death to any cause (PK, death trap, hell trip mishap, wanted flag suicide, whatever) auto-destroyed your unholy weapon and you think you could do well with that, I can recommend lich for you. Otherwise you're probably going to be a much tougher mummy.

So there's that to consider as well.