Seems like the go-to source on this is Fitzgerald, "Women Deacons in the Orthodox Church".
There was no official decision to eliminate the ordination of female deacons. I've seen dates saying the decline happens anywhere between the 6th and 12th centuries, and suggested reasons include:
Originally female deacons assisted female baptismal candidates, but the rise of infant baptisms reduced this need.
Possible reaction against the prominent place of women in early 'Gnostic' sects
Reaction to Islam and its stricter segregation, in part reflected in Byzantine society
Possible influence of the Western church after the fall of Byzantium,
Revival of levitical influenced laws regarding menstruation and uncleanness.
On the other hand, it seems that female deacons have always been ordained in the Japanese Orthodox Church, from the mid 19th century onwards, and may always have been ordained in the Russian Orthodox church. Since 2004 the Church of Greece has officially restored the practice (for the later: http://americamagazine.org/node/147165).