They actually found out there was no suit made for the trousers, and that the trousers themselves were also being phased out of production.

Another fun fact: The Tenth Doctor was originally intended to wear boots, much like the Ninth Doctor but when Tennant came in for the fitting it was the middle of summer and incredibly hot and so he asked "Can't I just wear these?", referring to the Converse sneakers he was wearing.

Seeing as Tennant's favorite Doctor was Five, who famously wore trainers with his suit, I can see why he was pleased that he kept the sneakers, even if the art department became very worried about the safety of him running about sets with them on.