I thought that wooden shields were mostly used by those who were less fortunate than others. What advantages do a wooden shields have over iron shields that made soldiers use them?
Especially for a shield, which must be held up in front of a soldier by the arm at all times, weight is a serious issue meaning that a good shield typically needs to be made either very small or out of very lightweight materials. During the later middle ages the iron buckler started to gain popularity as it was small enough to hang from a belt of a soldier using a bow or a polearm as their primary weapon, but for the most part shields made of wood and leather offered very good protection against spears, axes and arrows and metal shields remained very rare during the middle ages.
During the 16th century larger metal shields (called targets, often designed to be "proofed" against lighter firearms like pistols) did gain some popularity. However most foot soldiers during this period already took issue with the heavy armor they were expected to wear everywhere, never mind a heavy shield on top of that. As a result the metal target remained a relatively niche weapon. Fourquevaux wrote in 1548 about targets: ". . . there is but little accompt made of it, except it be for some assault, neither is there almost any man that will lade himselfe with it, except Captaines [i.e. those who could afford a page to carry their heavy shield around]." Sir Roger Williams, writing towards the end of the century, concluded that "their weights are such, that few men will endure to carie them (if they be of good proofe) one houre." He concluded that there was no need to have more than 200 targets of proof among 10,000 pikemen, and that it was best for the targets to be lightweight or of a thinner proof than the infantryman's breastplate or helmet already was.
The problem is with the word 'better'. Pre-gunpowder, wooden shields were perfectly adequate at doing what they were supposed to do. Attacks had to go 'around' shields, they didn't go through them. So iron would be harder to go through? So what? There really wasn't a class of attacks that iron would stop that wood would not. So no reason to bother with the great expense and weight of an iron shield.