Scientific writing style is not always very good at highlighting the unknowns. "We don't know this" doesn't make very convincingly looking text, so people tend to avoid admitting it up front.
But you are, of course, correct to ask.
Like another comments said, this is an open question.
One theory is, that while the algae floating in water were absorbing broad spectrum, the algae growing attached at the bottom of the water evolved to chlorophyll to capture whatever was left at the edges of the spectrum. And then later land-based plants would have evolved from the water plants that were already attaching themselves to the bottom. But then why are also the current ocean-floating algae green now?
http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/imgbio/pl...
Another theory is that a perfectly-absorbing leaf would somehow absorb too much energy and get overheated, and that it was better to absorb only part of the available light.
None of these theories are fully convincing, so the question remains open.