Who Needs Fenders? wrote:

A maximum of about 425km is due to the rendezvous operational limits
of the Soyuz (Soyuz can hit 460km, but is lower due to fuel
consumption during rendezvous). Add the heavy-lifting limits of the
shuttle and you see the altitudes stay down in the mid 300km range.

There's also the inner Van Allen Belt to contend with; get it much
higher that it's at right now and long term radiation exposure for the
crew becomes excessive.

Pat