Not anymore, though. From now on anyone can use Microsoft's assistant on Android, provided they don't mind beta software. The beta has gone public, but you need to be in the US to actually get it.
If you're interested, here's the download link. You have to agree to become a beta tester before you'll see the app in the Play Store.
Cortana for Android has most of the functionality seen in Windows 10 or Windows Phone. You can set and get reminders, search the Web, track flight details, and start and complete tasks across all of your (Cortana-running) devices. It's even possible to set Cortana to show up when you long press the Home button, if you feel the need to replace Google Now from that spot.
There are still certain things Cortana for Android doesn't do. It doesn't toggle settings or open apps, and you can't invoke it by simply saying "Hey Cortana". These features may arrive in future updates however.