Why do Eurostar trains have heightened security requirements compared to every other train in Europe?

While the Eurostar trains have a more strict security system than most of the trains in Europe, it is not the only one with security checks. The Thalys trains from France have extra checks, including a luggage scanner and a metal detector (as far as I can recognize the machines when passing through the system.)

Paris was attacked a few times in a short span and the powers that be decided that extra measures were needed.

So while most trains in Europe are not deemed to be under high risk of attack some are, and there are more trains with security measures and with the platforms of other Thalys stations changed for the Eurostar security, I would not be surprised to see the Thalys measures rolled out wider as well.

When the Channel tunnel was build to logical thing would have been to use it to properly link the Continental and British networks. However there were issues with that. One was that trains were out of fashion in the UK at that time. So Eurostar tried to look as unlike a train service as possible. Hence the compulsory check in, the separate terminal, the security check etc...

Another argument is that the channel tunnel is special, and thus requires these security measures. However, the rules actually do not require that all luggage is scanned. Only spot checks are required. The same applies to cars and trucks where only random checks are performed. The checks are also not know to actually work. We have not seen any thwarted terrorist plots, but we had two cases of the tunnel being shut down after a fire. Terrorists, take not. Taking out the Chunnel is actually rather trivial.

It is unfortunate that this security theatre exists. Apart from being inconvenient and not serving a purpose it also hampers the expansion of Eurostar services. You can now go non stop from London to Amsterdam, but not the other way round. The Frankfurt - London service Deutsche Bahn wanted to introduce got shelved when they could not get the requirement for security checks waived. Services to many other destinations that would be very useful (Geneva for example) will never see the light of day. It has made the idea of commuting between France and the UK impractical. Compare this with that other great water crossing, the Øresund crossing between Denmark and Sweden.

There are some other railways that now also have security checks. But it is there even more obvious that they are only for show. Only checking passengers at some stations is pointless, as someone with bad intentions will just board at a station without those checks. The fact that so few incidents happen on trains shows us that in reality the problem that is "solved" here doesn't really exist.