Why was it difficult for Bruno to change his clothes? Chap 19 & 20

It was cold, muddy, and the pyjamas smelled.

Bruno took off his overcoat and placed it as gently as possible on the ground. Then he took off his shirt and
shivered for a moment in the cold air before putting on the pyjama top. As it slipped over his head he made the mistake of breathing through his nose; it did not smell very nice. 'When was this last washed?' he called out, and Shmuel turned round.

"I don't know if it's ever been washed,' said Shmuel. (page 204) 'Turn round!' shouted Bruno, and Shmuel did as he
was told. Bruno looked left and right again but there was still no one to be seen, so he began the difficult task of taking off his trousers while keeping one leg and one boot on the ground at the same time. It felt very strange taking off his trousers in the open air and he couldn't imagine what anyone would think if they saw him doing it, but finally, and with a great deal of effort, he managed to complete the task.