That's either a deliberate change by Google, or a bug in the Play store, and it's been going on for a while. I used to install my apps from Firefox on my desktop (it still let you install them to your choice of phone), but that stopped working a while back. Now it works once in a while. I guess they know about it and will eventually get it fixed.
Welcome to Android Central! Which device? We've seen a few posts lately about installations from Play Store taking an unusually long time. Have you already tried clearing the app cache and data for the Play Store app and the Download Manager app in Setting>Apps? To find the latter, you have to tap Menu>Show System.