The entertainment landscape is too cluttered today. Star Wars was alone at the top of the heap back in the day...
Nowadays you have LotR, Harry Potter, Twilight, various other franchises. All of them have decent sized fan-bases but the end result is each getting just a slice of the pie, whereas Star Wars had the whole pie.
My feelings chime in with this. From what I can tell, the big franchises recently are those you've listed, plus the Matrix.
But none of them have had the widespread and enduring impact of Star Wars (or indeed Star Trek). And I do think you're right to assign the reason for that to the decreased mindspace available for any new entry. There's just too much data out there, with too many delivery channels, all capable of individualisation, for any one franchise to really dominate any more.
Blockbusters will still happen, franchises will still bloom, but their impact craters fill in much more rapidly with new materia.