Don't know why, as in the CATOs, other patterns of camouflage is permitted for wear in the field. You would wear ACU in Universal Pattern, or Tigerstripe, hell you could even wear multicam, Finland M05 Woodland pattern or German Flecktarn/Tropentarn and no Reservist or CIC Officer has any right to chew anyone out because it is permitted in the Orders.
CadetPAT is CadetPAT and is not MARPAT. The US Military and Marines got their digital patterns after we gave them the technology to make their own.
Have you ever worn it? The Gen IV CADETPAT (which doesn't look anything close to a marine, not that it ever has) is much more durable, breathable and quick-drying. It's also stain-resistant (proof almost).