Because phosphines tend to be sigma donors and pi acceptors (at the same time). Water, on the other hand, is primarily a sigma donor, so it is a (MUCH) weaker-field ligand on the spectrochemical series than phosphines are, and cannot displace phosphines that are attached to a transition metal.
(Two other similar ligands in behavior are "CO" and "CN"^(-).)
A sigma (sigma) donor is a molecule whose highest-occupied molecular orbital (HOMO) is a sigma-type orbital, formed from two orbitals totally symmetric about the internuclear axis. Examples are overlaps of np_z atomic orbitals, (n-1)d_(z^2) atomic orbitals, etc.
sigma donors tend to come in and donate electron density into a sigma^"*" orbital.
A pi (pi) acceptor is a molecule whose lowest-unoccupied molecular orbital (LUMO) is a pi^"*" type orbital, formed from two orbitals that overlapped sidelong. Examples are overlaps of np_y atomic orbitals along the x axis, (n-1)d_(xz) atomic orbitals along the x axis, etc.
pi acceptors tend to help stabilize the compound by accepting electron density into their pi^"*" LUMO.
Phosphines, however, are both of those at the same time. For example, triphenylphosphine, Ph_3P-, have this capability. Consider R = Ph for the following:
bbsigma donation:
)
bbpi acceptance (i.e. backbonding):
)
The phenyl groups would help the pi acceptor ability of the phosphorus, since they can redistribute the delocalized electron density better than, say, if R was alkyl, thereby stabilizing the bond order of the P-C bond (thus counteracting the fact that electron density in the antibonding orbital increased, which would have weakened bond strength).
Water does not have the ability to pi backbond, so a phosphine's bond with a transition metal is more stabilized, and it is more resistant to incoming water ligands in a displacement reaction.
