There are some reasons, this URL supplies some of them.
Especially note the repair risks and the liquid styrene risks


http://www.redrockstore.com/resin.htm


A short reprint of the major points follows, but a review of the total
article suggested


Epoxy resin is known in the marine industry for its incredible
toughness and bonding strength.
Quality epoxy resins stick to other materials with 2,000-p.s.i. vs.
only 500-p.s.i. for
vinylester resins and even less for polyesters.
In areas that must be able to flex and strain WITH the fibers
without micro-fracturing,
epoxy resins offer much greater capability.
Cured epoxy tends to be very resistant to moisture absorption.
Epoxy resin will bond dissimilar or already-cured materials which
makes repair work that is
very reliable and strong.
Epoxy actually bonds to all sorts of fibers very well and also offers
excellent results
in repair-ability when it is used to bond two different materials
together.
Initally, epoxy resin is much more difficult to work with and
requires additional skill
by the technicians who handle it.



Vinylester resins are stronger than polyester resins and cheaper than
epoxy resins.
Vinylester resins utilize a polyester resin type of cross-linking
molecules in the
bonding process.
Vinylester is a hybrid form of polyester resin which has been
toughened with
epoxy molecules within the main moleculer structure.
Vinyester resins offer better resistance to moisture absorption than
polyester resins
but it's downside is in the use of liquid styrene to thin it out (not
good to breath that stuff)
and its sensitivity to atmospheric moisture and temperature.
Sometimes it won't cure if the atmospheric conditions are not right.
It also has difficulty in bonding dissimilar and already-cured
materials.
It is not unusual for repair patches on vinylester resin canoes to
delaminate or peel off.
As vinylester resin ages, it becomes a different resin (due to it's
continual curing as it ages)
so new vinylester resin sometimes resists bonding to your older
canoe,
or will bond and then later peel off at a bad time.
It is also known that vinylester resins bond very well to fiberglass,
but offer a poor bond to kevlar and carbon fibers due to the nature of
those two more exotic fibers.
Due to the touchy nature of vinylester resin, careful surface
preparation is necessary if reasonable adhesion is desired for any
repair work.