Because it was cheaper than a silver mirror, originally
One of the things the D&D 5e tried to do during development was “unify the editions” somewhat. If you reach back to 1974 and the Men and Magic book’s equipment list (page 14), you find:
Steel Mirror / 5 gp
Silver Mirror, small / 15 GP
In play, the steel mirror was more durable, but depending on whom your DM was, it may or may not have been “as good” as a silver mirror. The biggest benefit when originally outfitting your dungeon delving character was that it cost less, which allowed you to perhaps by better armor for your character or a few more flasks of oil. Everyone rolled the same 3d6 x 10 for starting gold, so having to pick your starting equipment was a case of shopping on a budget.
Each player notes his appropriate scores, obtains a similar roll of three dice to determine the number of Gold Pieces (Dice score x 10) he starts with, and then opts for a role. (Men and Magic, p. 10)
In mosts games that I played in that era, they were functionally identical. The amount of verisimilitude engaged in at a given table will inform how a DM chooses to differentiate them functionally, if at all, in D&D 5e.
Why was that distinction made? The game was allegedly set in some vague “medieval time” but with the swords and sorcery literaty genre, Renaissance era norms and trope are not uncommon to find as well as anachronisms.
Glass-fronted (silvered) mirrors existed back to Roman times, but were
not common. Polished metal mirrors were much more common during the
middle ages. (Thanks @Blckknght)
Note that historically decent glass mirrors appeared in Renaissance.