The reason: "The first thing that makes running Just Landed difficult in the long-term is a serious lack of innovation in the flight data industry. An app like Just Landed relies on access to high quality flight data to function correctly. However, since the airline industry is extremely fragmented, and uses antiquated IT systems and many incompatible data formats, it is not practical for a small independent developer like us to negotiate data-sharing contracts with each individual airline, and then unscramble their jumbled data feeds into a usable format at a reasonable cost."

and a secondary one: the app ecosystem isn't worth it anymore:

"With well over 2 million apps by now (officially 1.5M as of July 2015), the iTunes App Store is an incredibly crowded place where it’s almost impossible to get noticed. Despite the persistent myth of the app developer millionaire, it’s extremely hard to make a profit — let alone a living — as an iOS app developer. The Google Play Store is a similar story, except with the added bonus of rampant piracy and a zillion devices to support. There really isn’t gold in them hills, at least not anymore, and independent app development will soon be in sharp decline, if it isn’t already.

Essentially, there’s a massive oversupply of apps, and the app markets are now saturated and suffering from neglect and short-term thinking by the companies who operate them. As competition has intensified, developers have resorted to giving their apps away, often at the urging of the app stores themselves. Consequently, the willingness of users to pay for apps, even unique apps like Just Landed, has declined sharply, while expectations of what an app should deliver have never been higher. Developers have been forced to get absurdly creative with their business models and marketing strategies just to make any income at all, rather than making the best product for their users. Just Landed, being a paid app with high running costs, has been on the wrong side of these trends, and what worked in 2012 no longer works in 2016."