A combination of many factors. (makes me sound knowledgeable...

)
Major contributor to this in my opinion is the internet and ripping the product and putting it out there for everyone to see without having to go to the movie theatre.
Actors, producers and so forth realise that there won't be much money made as it used to, so they just don't want to give a worthwhile movie just to lose out.
Think about this concept, you create a masterpiece and people reward you for your masterpiece, then what happens, it becomes copied and distributed openly.
What does one do now, continue making masterpieces receiving less and less each time, or just make mediocre standard stuff and reap the same rewards?
It is the same with countries and distribution of things like diamonds and gold, they control the distribution, thus keeping them precious, whereas, with movies,
no one can control the distribution, but again this can be disproved like below.
Another factor could be there aren't many good writers out there anymore, or the fact that every type of scenario has already been portrayed in hundreds of movies,
so anything new can be dissected and criticized more easily and thus movies these days are just crap in our opinions.
Another concept is the fact that we have become desensitized by the fact that everything can be found just using our fingers. Not sure where I was going with this, but,
I think it based around that we will become living zombies when it comes to the events of everything including real life drama.