Because there was a big problem at the time with people essentially worshipping angels and making up a lot of superstitious and possibly demonic prayers to angels.
In our present day this has made a bit of a comeback with all the New Age emphasis on angels.
It’s okay nowadays to depict unnamed angels in artwork, just as generic angels swarming around Jesus or Mary or whatever. There is still a prohibition on naming angels other than Michael, Gabriel and Raphael who are named in the Bible. Some churches do depict a fourth archangel named Uriel, who is venerated in the Eastern Catholic Churches, but not the Latin Church.
