Outside of the US, soccer fixtures are the biggest sporting events (certainly in Europe & Russia).

Due to a long history of violent football hooliganism, there is already elevated security around these events & international intelligence coordination preceding them.

Additionally, it is usually very difficult to obtain tickets unless you have connections to a club.

These factors would make them trickier targets than lesser sporting events or concerts.

Differences in transport access to sporting stadia & theatres in different countries may also play a role. Terrorists naturally take logistics into account.

And maybe the US just picks up more chatter about sporting events & not much about theatres?

But yeah, broadly either type of target is possible anywhere.