8GB internal storage is the real problem. Internal storage always matters, even with an SD card. You didn't notice with the S6 because that had nothing else. But a phone with only 8GB internal storage probably only has 3.5GB or less available to you (with the system firmware and cache partitions using the rest). Since apps' data live in internal storage too that will fill up very quickly. You can try clearing app caches - it's amazing how much space browsers and social networking apps can eat up with temporary files - but that's a temporary fix.
There are two ways of moving apps to SD, but I don't know whether either is supported by that phone. The first is the old version from android 2.2 & 2.3: settings > apps, select an app and see whether it has a "move to SD" button. Only user-installed apps can be moved that way, and not all of those (it's up to the developer). This only moves part of the app and not the app's internal data, so you still use internal space, just a bit less. This method was actually dropped in android 4, but many manufacturers kept it in their software - I don't know whether ZTE did. If the button is there for any app, even greyed-out, then your phone has that option for apps that support it. If it's not there at all for apps you installed then it hasn't.
The other method is "adopting" the SD card as internal storage. This was introduced in Android 6, so you've a chance as long as ZTE included it. This will probably be in the storage menu if anywhere. It lets you format the card the same way as the internal storage (so ext4 filesystem rather than fat32) and encrypts it, then will treat the card as part of the internal storage. Since this starts by reformatting the card all data on it will be lost (so back up first), and since it's encrypted you can no longer use it in another device. SD cards are less reliable than internal storage and this will increase card wear, so keep your backups up to date.
If your phone has neither option, then the answer to the question "why not?" is "ask ZTE".
But as the first reply said, nobody should consider a phone with less than 16GB internal even with an SD card unless they plan on using it purely to makes calls and send texts. And I've no respect for a company that still sells such phones, whether the manufacturer or the carrier, because there are many people who won't understand the implications. I'm sorry you got caught, and hope there is a solution.
