Threads tend to get locked for a variety of reasons, including:
1) The question that was asked has already been sufficiently answered, either on this board, in that thread itself, in some text, or elsewhere, so it doesn't need to continue to be re-hashed. Once the case is closed, to speak, so's the thread.
2) The question is invasive, cheeky, or stupid, in that it's already been answered sufficiently answered somewhere, it addresses a topic that is none of the inquiring party's business, it's so dumb it doesn't warrant a reply, or it was asked with the intent to deliberately and passive-aggressively shit disturb instead of sincerely obtain information.
3) The thread turns into an "emotional black hole" of off-topic badgering and fighting, in which case the conversation loses purpose and direction.