Because policies on health are best handled under specific health legislation, not under trade legislation, we dont specify how many non EU tomatoes can be sourced under a bill to build new hospitals. It was a wrecking amendment designed purely so the opposition could then shout on about how awful the government were for voting against it and not protecting the NHS
and therefore clearly selling it to the highest bidder, just like they did with their nurses pay amendment and most of the people shouting loudest about how awful it was think legislation is designed just like voting on some social media poll.
The NHS doesnt have to be protected within trade legislation for it not to be on the table when discussing trade.