Super-regeneration is an old pre-war amplification technique. It involves feeding the same signal through the same amplifier repeatedly (positive feedback) to amplify it by a great amount and then cutting off the signal (quenching) before it gets locked into oscillation. It produces very high amplification but can be a bit unstable and does tend to amplify noise a lot. One of Rife's descriptions of one of his early machines says that it used super-regeneration. But that statement alone doesn't tell us much about how it was used and in what context. In the absence of further information we can only assume it was only used for amplification and there are much more efficient modern techniques that are used in modern machines. So the reason it isn't used is because the only thing we can specifically assume it was used for in the past, can be done today using different and more modern techniques.

There is also the possibility that Rife was confused about the use of the term "super" regeneration as opposed to simple regeneration. Even many professional electronics engineers are confused about the difference and so this is a feasible possibility. It is undeniable that Rife's machines were regenerative (i.e. used internal oscillation) and it is probable that Rife used super-regenerative detectors for measurement (a common technique in the 1930's), so it is understandable that Rife may have misquoted the role of super-regeneration in the machines.