Its apparent synergy with the Standard powerhouse Goblin Rabblemaster was never realized in Standard. Perhaps being three colors make it difficult for many decks to cast, but three-color decks were common in those days. Honestly, this enchantment is fairly powerful, so why did it never see more play in Standard?
Making a 1/1 Goblin for each non-token creature you control that attacks is pretty strong. Its second effect, giving your team a +0/+3 boost until end of turn by sacrificing the Ascendancy seems pretty good, too. Swing with the team and not worry about losing anyone to blockers? It could be an effective way to survive removal like Bile Blight or Anger of the Gods.
It would seem the primary reason that this Ascendancy didn't see more play in Standard was that it was competing for deck slots with another three-drop from Khans of Tarkir, Hordeling Outburst. The Outburst creates 3 1/1 Goblin tokens straight out for 3 mana. For tempo reasons, the immediate 3-for-1 proved more efficient. Still, Mardu Ascendancy can produce grindy mid-range value. Still, it can be a pretty lousy top-deck in an aggressive deck. Three bodies on demand has its appeal.
The good news for Mardu Ascendancy is that it power has been realized by a few popular Commanders in EDH. This Enchantment has become an all-star in Alesha, Who Smiles at Death decks, and is a key contributor to many Zurgo Helmsmasher and Queen Marchesa decks. Being a three-color card in EDH really limits the number of homes this card can have. But if you're ever playing a deck in these colors that features a whole bunch of little guys, this is definitely worth considering.