To bring a five year old, increasingly shabby set up to high definition standards mainly.
If that's the officially stated (or leaked) reason, fair enough. But otherwise, I don't really buy it. This show isn't the original series of Star Trek, designed for flickery and unsharp 480-line screens. It's a 2005 show, designed with crystal-clear PAL reception in mind.
Besides, the special effects and sets on NuWho have seemed, at times, almost intentionally crummy and low-budget (the fourth season finale was worst in that regard), and I've no doubt that the same will occasionally be true for season five. The TARDIS interior was never the weakest link, and often the strongest.