Why Microsoft still believes in Windows Phone even though things have been pretty bad for years

It's been the same story for the last few years.

Microsoft's Windows Phone operating system is universally accepted as a solid alternative to iOS or Android, but it's still missing the key apps people love to use.

And because of Microsoft's tiny market share in phones, a lot of important developers pretty much ignore the platform.

But, Microsoft still believes in Windows Phone, and its next answer comes with Windows 10, the new operating system that will run on all Microsoft gadgets from the Xbox and tablets to PCs and phones.

Windows 10 will also allow developers to write an app once that can run across all devices, no matter what the screen size is. This, in theory, will give a boost to the quality of Microsoft's app store for phones.

"We're coming into a world where people aren't thinking about a disconnected category of devices, just about what they want to do," Greg Sullivan, a senior director for marketing in Microsoft's devices division, told Business Insider at the Mobile World Congress in Barcelona. "They are not these discreet categories of devices. They're a spectrum of devices."

Translation: In Microsoft's view, people care less and less about what device they're using. Instead, they just want to email, share a document, use Facebook, whatever. And the company that can make those experiences seamless from device to device will win.

Sullivan pointed out there are over 1.5 billion Windows users in the world. Many of them will be eligible to upgrade to Windows 10 for free later this year. When that happens, Microsoft is betting app developers will jump onto the platform and reach a massive market almost immediately. Eventually, the developer interest will trickle down to Windows 10 phones, too.

Microsoft also plans to launch a premium flagship phone around the time Windows 10 launches, Sullivan said.

It's still a big bet. Tablet and PC apps work very differently than mobile-only apps. There's very little chance the developers behind Snapchat, Instagram, and whatever the next hot app is will take Windows 10 seriously just because it's running on more than 1 billion PCs.

Still, Microsoft will share more about its plans at its Build developers conference in April.