In the end, those questions beg the much more attention-grabbing one: how was once Rey in truth in a position to summon this Force lightning by chance within the first position? It is in all probability strictly due to one thing with heritage, as it’s frequently emphasised in “Star Wars.” Being a Palpatine, Rey most likely already has some more or less genetic predisposition to Force lightning given how a lot Sidious used it. For the reason that we do not know a lot about her oldsters or in the event that they even used their predispositions to the Force, it was once most definitely handed down regularly. On the other hand, given Kylo and Rey’s immensely robust connection as a Force dyad, it’s unexpected that Kylo would now not be in a position to summon Force lightning unintentionally both. Perhaps he can, it might be conceivable, even if he has no lineage of Force lightning customers.
Both means, we most likely won’t ever be in a position to verify the precise reasoning as to why Kylo cannot or may not use Force lightning, particularly seeing as he returned to the Gentle Facet, whose customers by no means faucet into that skill — and the truth that he’s now lifeless. On the other hand, it is going to be attention-grabbing to see if or when Rey reappears, and whether or not that Force lightning will reappear deliberately or unintentionally all over again.