You're not "buying" it? Really?
YOU might not be finding something else on - but clearly millions of people are. Crap or not - people are watching. And if they're watching that - they aren't tuning into any big "event." Plus the internet and other distractions that didn't exist in the 70s..
Not acknowledging that, to me, is pretty silly as it's the obvious factor.
Roots WAS successful and remains so because of the acting, direction, script, etc. But you can't discount external factors either.
Another example is the the original Star Wars trilogy. No one had seen anything like it. It was a phenonmenon. So much so that it was constantly in the papers, magazines and on TV. The twists the plot took (Luke I am Your Father) - I recall quite vividly being a true cliffhanger at the end of Empire - so much so that everyone was anxiously awaiting Return of the Jedi. Anyone and everyone was talking about the series. It was on the cover of Time Magazine at least once if not more I recall. The topic was on everyone's mind - or so it seemed.
There are very few movies that can or have captured the same excitement universally. You can argue the Harry Potter movies - especially now with the internet and the fan base. But to me it doesn't feel the same. Perhaps it's because the fans have all read the books so they know what to expect. There's no shroud of mystery about twists and turns.
There are many factors which come into play when determining success. If there was a magic bullet - we'd see more of it...
"A little nonsense now and then is relished by the wisest men" - Willy Wonka