A bit of the low sales with Solo is its release time. Within a month or so either side of it there was Black Panther, Avengers Infinity War and Deadpool, all three of which were box office gold and collectively spanked everything else released in the first half of 2018. Regardless of the 'universes' each of the films is from, most Star Wars fans will go to MCU films and whatever Deadpool is from (MCU too? I dunno), so too many mega blockbusters released at the same time - blockbuster fatigue? Star Wars films usually come out in December, so Solo was misplaced in that respect. I still haven't seen it, so can't hazard a guess as to whether or not it might have been a success had it been released in December.