Did they say when reunification happened? The singularity tech may have been ditched in favor of M/AM drives for whatever reason (inefficient, not enough resources to build them, too many reliability issues, etc.). Doesn't explain why they couldn't build a new one, although with the Vulcans being famously cautious, it may still be in "re-development".I am curious now - what ever happened to Romulan tech that used an artificial singularity to power their warp drives? They didn't use M/AM or dilithium crystals at all to my recollection. However, the Ni'Var all seemed to be affected by the Burn and its attack upon dilithium. Were the singularity drives all broken as well? Something doesn't track there.
EDIT: Just had another thought - the Federation should also have access to that tech, since Ni'Var was part of the Federation at the time of the Burn. The Romulan faction even wanted to stay part of the Federation. So yeah, head scratcher.