Because when people devised units, they didn't realize that there are equivalence relationships between length, mass and time.
But to more directly answer your question, Planck's constant really has units of (energy)(time). To see this, note that Planck's constant can be defined by the ratio of the energy and frequency of a photon :
[tex]h= \frac{E}{\nu}.[/tex]
In order to explain the relationship of energy to units of mass, length and time, you might want to consider basic formulae from classical mechanics.