[sally]

why the ketone form is prefered over the enol form( see the equilibrium in the figure)?
my idea is that the hydroxy group and the benzyl group cause steric effect in the enol form?

[Dan]

Sterics may well be be a factor, however I think the main factor here is the strenghth of the C=O bond.

Using typical bond enthalpies from here http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html
you can see that the ketone is lower energy.

enol ---> ketone is about -140 kJ/mol using those values.

[movies]

In this case you also have to account for the energy of conjugation between the C-C pi bond and the ester carbonyl.  Also, if you draw the other alkene isomer you could have an intramolecular hydrogen bond which would drop the energy by a little more, so the two are closer in energy that you would get just from the bond enthalpies.