PAK-FA already has an IRST. Perhaps they just ran out of room in the nose with the radar installation? Most strike missions probably use INS/GPS anyway. When I see footage of LGBs hitting stuff it usually is a CAS situation, and presumably you wouldn't do CAS in contested airspace.
This. Albeit not exatly "ran out of space", but just never intended to have this system inside. T-50 was always multirole with AS as main role, unlike F-35 which is same multirole but with strike mission as head goal.