Because (a) that's what they were given to wear, and (b) it worked. These two points are related - if the armour didn't work, they wouldn't have been given it to wear.

First, there's the question of timing. Before plate armour had become reasonably common, soldiers wouldn't have worn non-torso plate armour. This puts the start of this brigandine-plus-plate period at about 1400. Later, plate torso armour became standard, and the brigandine became less common. This puts the end of this period at about 1500, as discussed by u/WARitter in https://www.reddit.com/r/AskHistorians/comments/5182sk/were_cuirasses_common_for_foot_soldiers_during/ Note that this is a predominantly infantry combination. of armour - armoured cavalry were wearing plate torso armour during the 1400s.

Second, there is the question of "why?". We can note that brigandine works, and works well. It appears to provide better protection against arrows than a plate breast-and-back of the same weight, as I discussed in https://www.reddit.com/r/AskHistorians/comments/aq2oi7/what_aspects_of_a_brigandine_make_it_more/ Further, since a brigandine is somewhat flexible, it can offer better comfort than a munition plate breastplate.

These last two points suggest that perhaps we should ask the question the other way around: why was the brigandine replaced by plate torso armour c. 1500 as the usual infantry munition armour? It is possible that this was driven by the arquebus, which developed during the 15th century and became more common on the battlefield over time. Whatever advantages brigandine might offer against arrows, a solid breastplate was likely to have been a better choice when facing guns.