Welcome Bloodnut. No real reason - Bluetits check out lots of different locations but many birdboxes remain empty at any one time, even when they have been used before. Since they produce several broods, I wouldn't give up hope yet of a pair taking up residence.
Welcome to the forum Bloodnut hope to hear more from you..
I can't add anything to what GD has already said.
Blue tits usually only have one brood a year but some years you may find that birds that try early and fail may give it another go a bit later, it's still only just the first few days of May so you never know, things could still happen...