Why is quantum physics needed to explain photosynthesis?

The absorption of light is a quantum process.

Here is an article from Nature (2004) that addresses the question:

http://sitemason.vanderbilt.edu/files/iqLVYI/Fleming 2004 Nature 431 256.pdf

Well, photosynthesis has to do with the absorption of photons to break/make certain bonds. The most classical you can get with this is Bohr's model, nothing prior to that explains absorption of light. Bohr's model is kind of an extremely basic type of quantum mechanics, and anyways it does not work for multielectronic species. Bonds become impossible to analyse here.

Normal QM/molecular orbital theory, on the other hand, deal with the nature of the electrons, even when they are in bonds. So it explains the process pretty well.

So it's not that some classical theory failed, it's that there was no way to classically explain photosynthesis further than "the bond absorbs light". Why does the bond absorb light? Because it can{*}.

Classical theories need not fail in every aspect :)

^{*Insert appropriate James Bond joke here}

The article Arnold links to is well worth a read, but in addition to that, recently there has been a lot of interest in the role of quantum entanglement and coherence in photosynthesis. If you've recently come across articles about quantum physics and photosynthesis it's probably this aspect that was being described.

I don't know of any definitive review of this area, but Googling for "photosynthesis entanglement" or better still "photosynthesis entanglement site:arxiv.org" finds lots of articles.