I think it has to do with the changing perception of short scale basses.
To elaborate further, if you look back to the 70s a lot of budget basses were short scale, there was Kay, Jedson, Columbus and a couple of others, pretty much all of the output from Kay, Jedson and Columbus was poorly made junk, microphonic pickups, cheaper than cheap machine heads, dreadful bridges, terrible electrics.
Even Fender during that period in time seemed to treat the short scale with a degree of contempt with the (IMO) dreadful Musicmaster bass that was usually put together from the leftover parts from other models like for example the strat pickup that was under the black pickup cover.
Then there was the false perception that short scale automatically meant mud, this was largely a result of the kind of basses that were short scale that were available , the above mentioned basses were tonally limited due to their cheap electronics combined with the flatwound strings and the poor amplification of the time, this perception was not helped by the Gibson EB0 and EB3 that had massively overwound pickups and the trend of those times for short scale basses to be hollow or semi hollow.
Fast forward to nowadays and the sands of time have helped for those perceptions to be largely forgotten, a lot of the people now playing bass don't have memories of learning on those dreadful cheap basses of the 70's and the short scale basses that are around nowadays are far superior because to put it simply people expect better nowadays and it is now possible to make better instruments nowadays and so people are trying these basses and finding that they are actually more comfortable to play than long scale basses (for most people ymmv etc) and actually sound pretty damn good which of course drives demand for more short scale basses.
Controversial perhaps but I would be willing to bet that the Squier Jaguar ss bass has played a large part in the resurgance of the short scale