It was the traditional military outfit, that's why.
As to camouflage, um, that's nice and all, but there generally wasn't too much of a need for it during trench warfare. It doesn't matter all too much what you're wearing (except that it's durable) when you're absolutely covered in mud and grime and the only time someone has a clear shot at you is when you're running straight through the clear no-man's land, where they'd still notice a person in camouflage and mow them down in a hail of bullets just like the rest.
You have to understand that the war was less about mobility than many of the following ones, and so camouflage and staying hidden weren't so much the concern. It was known where the enemy was; the problem was that neither side could get the other to budge without sending over vast amounts of soldiers to be fed into the meatgrinder that was the other side's defenses, based on the hope that the machine guns and the like would be unable to kill enough people to stop the trench from being breached.
After that obscene cost in human life, of course, they'd only have gained a couple yards, and were just as likely to be fought back to the previous trench.
Anyway, that aside, they updated their uniforms mostly because the older fashion ones weren't really suited to the environment they were put in (a lot of time being wasted on clean-up, for example) and the newer style was seen as more practical to wear.