If you don’t know what that sounds like, it could be something like “we wanted you to take on that project but the deadline has moved forward”
It’s not you, it’s me.
Why do managers hide behind reasons out with their control? Or even worse use those reasons as an excuse. If you really want someone to be on a project then you will genuinely do everything in your power to get them on the project. And when you are using those reasons to cover up thinking that someone isn’t good enough then you are doing everyone a disservice.
Telling someone that they could have done A but unrelated B stopped them rather than saying they weren’t good enough to do A in the first place actually damages your relationship. You may have spared their feelings but without honesty you can’t effectively help someone to grow. And if you don’t help them to grow, they stop working.
Your opinion and decisions are what make you a leader. Nothing else. No industry knowledge, no strategy formulation, just your decisions. You may make decisions about strategy and you may have opinions on the credibility of knowledge which offer more to those that work with you, and for you, than just knowing.
So instead of thinking of reasons to tell your staff, let them know it’s your decision. You may need to justify it but at least your position will be clear.