Why was Segwit a soft fork?

I’m curious as to how the segwit update managed to propagate slowly and avoid being a hard hork without securing 50% of the hashing power.

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I did some searching, and found a link answering your question. I’ve quoted the main part of the answer below:

Witnesses are only included when the requester asks for them, which old clients don’t do. For them, new clients remove the witnesses before relay. Exactly because the witness of a transaction does not contribute to its txid, it is in fact possible to remove them before relay, without invalidating them. That is what makes it a soft fork.