Because you were using a DTH, and now you need an Edge driver. ST has started the migration away from Groovy for DTH’s and SmartApps, which is why you longer can add, see, or delete them in the IDE.
If you removed a device that was using a DTH that had not yet migrated to Edge, and tried to add it back, you’ll now be required to have an Edge driver for it, especially if the DTH you were using didn’t have fingerprints for the device properly set up. In the IDE you could force a device to use any DTH, but you can’t really do that anymore on the updated platform.
This is a very long discussion about the end of Groovy, but it has useful dates, links to other related posts, and should answer many questions about the migration. If you’ve not been up to speed on this platform change, I recommend that you take a look: