Part of the reason is Moon+ and part of the reason is the size. I like the narrow text, the size of a newspaper column. I think it makes reading faster, at least for me.
Moon+ also gives me very fine control of font, font size and the boldness of the font, as well as the color of the background and text. I use an amber background that's just one of the many ambers available. I really don't do much with the font except to make it a little bolder.
All of that is kind of nice but what makes Moon+ really shine for me is that I can scroll instead of turn pages. Each chapter is a very long page, much longer than the screen, and I just move the text up to keep reading down. I love that about it. I generally keep the top of my current paragraph near the top of the page and if I have to stop in the middle of a paragraph when I come back I know right where to start. I like re-reading from the beginning of the paragraph to bring myself back into the book. It has automatic scrolling also but I don't use that.
Another very nice thing about Moon+ is the way it measures progress. Actually it has a variety of ways to do that and I can choose the one I prefer. I keep it set to give me 2 numbers in parentheses like (a,b) where b is a number indicating the number of screens in the current chapter. That, of course, changes if I change the font, but I usually don't so it's pretty stable, and a is how many screens I've read into that chapter. This lets me see a very fine-tuned report of my progress, which I like. I usually try to make a session last at least to the end of my current chapter and this kind of pushes me a little and gives me an idea if I want to go ahead and read the next chapter too.
Moon+, like the Kindle, syncs my progress between devices using Dropbox. The problem with my Nexus is short battery life so now when I'm reading on the phone at home I keep my old, no longer connected phone handy to use while charging the Nexus. Syncing on this is much smoother and faster and simpler than on Kindles. Not that Kindles are bad at that, this is just a whole lot better.
It can also use my ebooks folder on Dropbox as an extended library. It can do that and also sync with OneDrive or Google Drive and probably others. I've been putting books on Dropbox for a while now just in case. I've never really needed them but it's often easier to get to them than to go to my computer so that's working out nicely.
When I had an Ipod Touch a few years ago I tried a number of ereader apps and found a few nice ones but nothing that scrolled. Scrolling is something that nearly all Palm ereader apps did and it's what I've always been used to. In fact my very first ereader, Vertical Reader on the HP95lx scrolled. I've never understood why people turn pages. What a silly idea!
Not long ago I read a review of some ereader for the Iphone that sounded pretty good and I can't think of it's name. I just googled for it but no luck. I do remember the review talked about it's scrolling abilities and made it sound pretty good. That's a favorite feature of mine.
When I had my Ipod Touch I used a number of reading apps and I think Bluefire was one of my favorites. I don't recall why. I have no idea if it's still a good one. At that time it was brand new. I did a bit of emailing with the author and it was obvious he was both serious and knowledgeable.
I'm hoping when my new phone with the Amoled screen gets here I'll be able to read in longer sessions on the phone. I don't expect to be able to abandon my Kindle but if I could that would be the most desirable outcome.
By the way, Moon+ has one downside, a mediocre dictionary. It let's you choose other dictionaries but the default one seems to be the best. It's as good as the dictionary on the Kobo or Nook but not as good as the one on the Kindle.
And, if anyone has an old Fire Phone (I do) the Kindle app in that scrolls, and very nicely too. I wish the other Kindle apps were as nice as that one. It has quite a few special features none of the others had. The Fire Phone in general isn't that great. it was obviously designed by a bunch of bright and eager 5 year olds but you have to give them credit for trying. They did get the Kindle app right. I also have Moon+ on the Fire Phone, where it works just fine, but I tend to use the Kindle app on it.