The Original Super Wasp didn't use Endo (It' hadn't been invented yet) but rather a custom skeleton that weighted half as much, my guess is that TPTB didn't want to go either of those routes so they came up with something different
<switches on debunk-urban-legends mode>
Only half true. It did not feature ES, but it also did not feature any lighter internal structure whatsoever. It was simply 1 ton overweight. The best explanation I've heard is that it reused the armour of the Battle Droids Wasp, which was 1 ton overweight, too.
Best I can tell, the legend about lighter internal structure can be traced back to my favourite loooooong dead website, "Wars in the Inner Sphere" which featured the infamous "Technical Readout: Legends" that presented apocryphal, abandoned, or fluff variant designs. It presented the ES solution.
My gaming group, for instance, always played it as a 8/12/4 design.
So clearly, another solution next to prototype ES was deemed more appropriate for this publication.
On the HS debate: some things change. Early incarnations of the Freezer rules had them at 1 crit, but they have been solidified into their current status in at least two publications.