Why are all the DC biasing sources assumed as ground in BJT transistor h-parameter equivalent model?
For small-signal analysis the transistor is represented by a linearized small-signal model. The linearization is done for a certain operating point and excursions around that operating point are analyzed.

Since the whole circuit is linear superposition can be used. Only one source at a time is considered and then the results are added. Fixed DC don't generate any changes, therefore their contribution is zero. It is sufficient to only consider the AC source.