It's a trend.
One company does it, two companies do it. Two companies do it, four companies do it. Etc.
Tim Cook said they wouldn't make a bigger phone but now with the iPhone 6 Plus, they have. Why? Because other companies have done it too. And as a company you sometimes have to go with market demand: users want it, so they have to make it.
Users switched, because if they wanted a bigger screen, they had to switch to e.g. a Samsung Galaxy.
Phones/tablets are becoming our PC.
No matter if you're working, sending some emails, browsing on Facebook... phones and tablets are taking over these tasks from the PC. Therefore, a bigger screen is required. You have to admit, sending documents on a 3.5" inch screen isn't that comfortable, right? ;)
They're still compact.
This trend is probably nowhere near it's end, because the devices are still compact.