From Connolly and Dodge, The Ancient City

The depiction of the cavalry process takes up by far the largest part of the frieze.  It occupies all of the west frieze and most of the north and south.   The cavalry did not play a crucial role in Athenian warfare, but the horsemen were young men from wealthy aristocratic families whose privileged status entitled them to take part in the procession.  Of course, men on horseback made a significant contribution to the pageantry of the procession.

A recent theory claims that the number of horsemen, which total 192, represent the 192 Athenians who died at the battle of Marathon, in which the Athenians defeated the invading Persians..  Of course, one problem with this interpretation is that this battle only involved the Athenian infantry, which is strangely absent from the frieze (if it was supposed to represent the Panathenaic procession).  The originator of the theory, John Boardman, claims that the Marathon dead are depicted as horsemen in order to heroize them.

Another possible reason for the prominence given to horsemen in the frieze is Athena's connection with the taming (bridling) of horses.