Why in oscillator triode is used instead of transistor?

I do not understand your question, oscillators CAN be built with either (and many other amplifying components). A few examples are:

  • triode tube
  • tetrode tube
  • pentode tube
  • beam power tube
  • pentagrid converter tube (cathode and first 2 grids form local oscillator)
  • magnetron tube
  • klystron tube
  • traveling wave tube
  • thyratron tube
  • neon lamp
  • point contact transistor
  • junction transistor
  • surface barrier transistor
  • field effect transistor
  • tunnel diode
  • unijunction transistor
  • silicon controlled rectifier
  • magnetic amplifier
  • FERRACTOR
  • operational amplifier IC
  • timer IC
  • logic inverter IC
  • etc.

Perhaps you meant why it was in some specific circuit that you don't give any reference to. If this is the case, I cannot answer without a reference to the specific circuit. One possibility for selecting a tube instead of a transistor to implement an oscillator would be that it must deliver higher power and/or voltage than a transistor is capable of.