What reason is cited by the authors for the difficulty with receiving approval for sustainability as a strategy?

Sustainability is too easily achieved and therefore does not provide a competitive advantage in the market.

In general, directors often need to see a demonstrable return on investment to justify the costs.

Sustainability is not a classic strategy in the mold of quality, cost leadership, or flexibility.

Sustainability is an inherently high$-$risk strategy and most boards are risk$-$averse.