Retrica's popularity can be summed up in one word: selfies. If somebody - let's call her Sue - needs a picture of herself right this moment, she needs an app with a minimalist interface that doesn't get in the way. And since Sue Somebody is working with less than optimal circumstances and a mobile phone with a crappy front-facing camera, she wants a wide range of filters that could mask how bad of a photo she's about to take. Retrica fits that bill. She's seen her friend using the app on iOS for a while now, but now it's available in the Play Store for her to download as well.
Sure, Retrica is yet another filter-centric, photo-editing app, but it's one with a slick interface and bright colors. On Android, it hides the status bar to make more room for the viewfinder, but it leaves the navigation bar intact. The UI is clean and rather sleek, but you may have to click around aimlessly on most of the icons until you get a feel for what each of them does. There aren't many labels to help out here.
Retrica is available as a free download, but it's one that comes with ads. You can remove those and get three extra packs of filters (containing 25 altogether) for $1.99.