While I have never played it (including Windows 10, which is basically the same thing) it is likely due to the platform it is on; mobile games are quite popular; in fact, the best-selling game of all time owes most of its sales to mobile (Minecraft has sold more copies if you exclude mobile/PE sales):
The best-selling video game of all-time is Tetris, a tile-matching puzzle video game designed and programmed by Alexey Pajitnov. Originally released in the Soviet Union in 1984 for the Electronika 60, it was later popularised upon its release for the Game Boy in 1989. The game has been ported to a wide range of platforms and sold in excess of 495 million units, 425 million of which are paid downloads on mobile phones...
Excluding mobile sales, the best-selling video game of all-time is Wii Sports with 82.81 million units sold, followed by Minecraft with under 82 million units (excluding over 40 million mobile downloads), while Tetris is the fourth best-selling with sales of 70 million units.
The popularity of mobile games is in turn because of the popularity of the platform they are on - enough mobile phones were sold in 2015 to give one to more than a quarter of the world population, a fifth for smartphones only:
Increasingly, a smartphone is the only computing device that many people have since they are getting to the point where they can replace PCs (most people do not need the higher-end hardware that PCs provide) and are generally cheaper - sales and usage of which have been declining in recent years.
I would think that part of the reason for the huge sales difference has to do with the price as well. PC/Java Edition $27 U.S. Dollars, Console Edition $20 U.S. Dollars, Pocket Edition $7 U.S. Dollars.
What is weird is that Windows 10 Edition used to be $10 U.S. Dollars before the 1.0 release, it's now up to $27 U.S. Dollars.
I like playing PE while lying down on my back. It's comfortable for my injured back. I used to play the Java and PS3 editions a lot more, before PE became well updated. Occasionally I will fire up the Win10 edition, but that's mostly for typing in commands then I go back to PE on the phone.