It's not really 'random', it's a deliberate order that ensures buyers can't complete any of the particular series in the collection until the end of the collection. The idea is to string you along for the full length of the run and keep you paying from start to end even if you don't actually want a bulk of the individual volumes. Odds are good you'll get a chunk of Marvel G1 upfront but then the rest is strung out over the next six+ years