most of the transition metal do not displace hydrogen from dilute acids. Why?

because they are below hydrogen in the electro chemical series and hence there reducing power is more.

Transition metals are usually placed lower in the reactivity series. Also due to transition of these elements to a more electronegative one. Due to this their reactivity must be getting reduced. Thus transition metals cannot replace Hydrogen from acids, mostly. Ecxeptions : Zn, Fe, etc.

Many of the transition metals are sufficiently electropositive to react with mineral acids, liberating H2. A few have low standard electrode potentials and remain unreactive or nobel. This nobel character is favoured by high enthalpies of sublimation, high ionization energies and low enthalpy of solvation. Consequently, transition elements are weak reducing agents and are less reactive. They are place below Hydrogen in electrochemical series and are less reactive than hydrogen.