Why are managers so reluctant to use subs at halftime?

Generally in football, managers tend to bring subs on after a hour or later? Yesterday I think we would have won that game if we changed at halftime. Especially Colwill he changed the game and drove us forward.

Hindsight is never wrong

With 5 subs, making changes at halftime is a lot more sensible, than previously.

If your changing tactics or team shape because somethings/someone's not working out that would be acceptable.At the moment Bulut seems to be changing like for like without effecting shape or tactics but I take the point about Colwill who only now seems to be shaping up.