I can think of one good reason:

Not everyone you might want to peer with on the West Coast
is at the PAIX.


While that is certainly true of PAIX, it is also true of MAE-West and
PacBell. Not everyone is everywhere. On top of that, the "quality" of
peering with a given peer may vary from exchange point to exchange
point, depending on how that peer is built out to each of the exchange
points in question - a factor that is difficult for the exchange point
operator to control (or perhaps even know).

Anyone considering an exchange point should do so in a well-informed
fashion. Bill Manning conducted an "Exchange Point Engineering" BOF at
the S.F. NANOG, the notes for which are at:

A lot of what it takes to be well-informed can be derived from that.