I know I am a couple years late to this thread but an answer to a question like this would likely consider careful thought. I suspect that the reason the show ended is for one of three reasons or all of them:
1. The show just wasn't as popular as it once was among it's targeted demographic, children. They probably thought for this reason, that it would be impractical to keep the show going after nine years of it airing and probably knew video games were the new means of entertainment.
2. A lot of the main characters have already reached their goals as someone else who commented already said. So them dragging the show on afterwards would probably not only be pointless but would probably result in burnout and seasonal rot like the Simpsons.
3. Finally, 85% of the people who watched the show are probably grown adults now, especially since the targeted age demographic is 4 to 10 years old. Most of them are likely teenagers or adults by this point and would likely enjoy different shows or web series.
These are the three reasons why I personally think MLP ended, these may not be a full list of them but this should give you an idea as to why the show ended at the very least. What you do with this information is entirely up to you.