why sometimes BJT is used as a power switch (because of low current, low voltage, low cost or related vbe, large current leakage?)
->Low on-state voltage drops
->High Current capability
->Relatively cheaper compared to MOSFETs with similar requirements
why sometimes MOSFET must be used as a power switch instead of BJT (because of high-load current, high voltage or related vgs, less current leakage ?)
->BJTs can carry more current than MOSFETs(typically)
->High Voltage is definite yes
->High Frequency switching with simple gate drive requirements(like a pulse will put-it in on state, unlike BJTs which require continues gate drive)
Do note that leakage current is not a big factor, if at all it is a factor, in selecting power switches. As dfullmer suggested IGBTs give you the best of both worlds i.e. with some limitations at the extremes. In the end its just a matter of the application at hand.