I think a huge amount depends on how your employer functions. My company is currently pushing development and internal moves, but that's not the same as it happening in practice. Having said that, it does indicate they're at least on board with the theory, which helps pave the way.
Do you have a development plan of any sort? This could be a formal one that leads to your annual performance review or however your company does it - but even if they don't work like that, you can have your own plan. Where do you want your career to go? What experience do you want to gain? What will it bring to the business?
If you can answer some of those questions, then it will give you a way of asking more tactfully even if everyone knows underneath it's because your current boss is duff. You need to focus on where you want to go to rather than where you're going from when you consider the "why" question. Look at things like the experience you'll gain, and how that'll help you in the future. You should also be aware of what skills and experience you have from your current job, which would be useful when transferred to the new role. Emphasise how the change would benefit the business.
(In my case, I suspect this will be comparatively easy, if I find out where I want to go, as my manager said yesterday, "Well, you're basically in the wrong job." Which was helpful. Not)