At a guess, because less spell casting would open class space for more Skill and Martial features?
Which isn't really necessary since attack bonus is based in level, rather than class progression. And since skills aren't based on points anymore--thank God--bards and rogues don't sacrifice combat ability to be skill monkeys. The bard knowledge and expertise features work well for that.
At first glance, it may seem that the bard is just as good a spellcaster as the wizard, but it's not the case. The spells per day is the same, but the bard lacks the utility that comes with the wizard's spellbook. The bard can use only the spells it knows. The wizard can learn as many spells as he can find in scroll or book form. Now, admittedly, the bard can poach spells from other classes, but, again, it is very few, and those will mainly serve to flesh out a bard's role in the group, which can vary from campaign to campaign. Also, those are "permanent" choices, rather than additions to a relatively unlimited spell selection.
Then there is the matter if the bard spell list itself, which lacks the breadth of function of the wizard or even sorcerer. It has a few spells usually considered divine, but not like the cleric or druid. It can use scrolls, but, again, is limited by its spell list to eliminate the chance of failure.
I currently play two bards in different campaigns. They are certainly good characters, but they lack the specialization that makes other classes the "masters" of their respective domains. Even so, that breadth of knowledge is itself a strength, which allows the bard to shore up any weaknesses in the group.
I've already made my thoughts known on the stereotypical trappings of the bard, but I'll sum up here: singing and playing instruments is flavor and not crunch. Foci can be replaced with a spell component pouch, and Inspiration does not specify music at all. Song of rest does, but since you're not in combat, it's a non-issue. A lot of warriors historically have taken up some form of art, and music ability or knowledge is widely considered the mark of a well-educated person.
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