Should I talk to him ? How can I ask why without questioning his decision?
Yes. You should not question about his decision, question about plan for your future work and assignments in way that "hints" about your interest.
If I were you, I'd probably start a conversation saying:
"Hey Boss, I noticed that we got a new work assignment / project which is based on XYZ and it's good. It just happens to be there that after the last movement of mine into the current project, I am learning PQR and since it's been 6 months now, I can say I grew certain level of fluency and a natural interest about working with PQR.
However, as you know, my primary skill in on XYZ and I was wondering about any possibilities of new work assignment with XYZ in near future? Also, If I am able to help in the existing project in any way, please let me know".
This should get the job done without "questioning" the decision about the allocation. Also, this will serve as a reminder about your skill-set, and your willingness to work with your preferred technology, in case it fell through the crack. In case there is a valid reason for not granting you the new task - you'll be enlightened about that, too.