His plays are loooong. Directing a full production takes forever; children could be born during such a process. Sitting through the uncut versions also takes forever (we're exaggerating ever so slightly). Did people during Shakespeare's time actually have better attention spans? Maybe they just had better seats and could nap during the soliloquies that just go on and on and on...and on. Well, people with box seats had better seats, but many others were just standing on the main floor. Surely, William would not want to be upstaged by someone dropping to the floor during his shows.
Our dear Will was a performer-director, too. If it's one thing directors and performers don't want, it's to put their audience to sleep. Audiences who don't like what you're doing don't return to the next production. That could mean a serious loss of money. So, perhaps Willie, to keep the masses happy, cut his own shows. In fact, (here's a tiny bit of research coming; not sure about the reliability of the source) according to Wikipedia "The "two hours' traffic" mentioned in the prologue to Romeo and Juliet was not fanciful; the city government's hostility meant that performances were officially limited to that length of time. Though it is not known how seriously companies took such injunctions, it seems likely either that plays were performed at near-breakneck speed or that the play-texts now extant were cut for performance, or both."
We must digress: watching Shakespeare at breakneck speed is hilarious. Probably shouldn't be done with the tragedies.
To summarize the reasons we conclude Shakespeare cut his own plays:
1. He would not want his audiences sleeping, and therefore not returning.
2. Nor would he want to be upstaged by anyone in the audience snoring.
3. Wikipedia says the plays had to be kept at two hours.
4. Performers could seriously mess up the lines for such long and rhythmical productions.
Disclaimer: This article had almost no official research. It, like many of the other articles found on the web, might have a bit of truth to it. Maybe not. And just for the record, we enjoy Shakespeare.
If you're looking for shortened Shakespeare, Learning Links has them in various cuts for children grades 1-12 to perform. If you or your children just want to know what the heck the story lines are in Shakespeare plays, head over to Amazon and look up "Tales from Shakespeare." There are numerous books that create stories out of the plays. Once you look up the titles of the books, you can head over to your local bookstore and get those or similar books--you know, keep the local economy growing.