Why can't my z10 open a apk for instagram?

I downloaded a apk version of instagram over a year ago and I was very pleased with it. I didn't have to slide load anything. It worked just like that. (I had OS 10.2.1 at the time). Last night I accident deleted the app. When I click on the apk file (which is still in my file manager under downloads) it says that the file cannot be opened. I basically tried downloading a bunch of other apk files of instagram off Google but my phone can't open any of them. I'm currently on OS 10.3.1.1779. Does anyone know why my phone can't open the files and how I would be able to get a hold of a functional apk file for instagram that will work on my phone without the trouble of side loading, etc? (ps I don't like Igrann so please don't suggest I get that instead). Thank you!