They wouldn't. You're building miles of thick stone walls exposed to the elements, which you are deliberately forcing plants to cover as much of them as possible. The article and the followup hint at the repair and labor costs (note that the article is set in China):
> The decline of the European fruit wall started in the late nineteenth century. Maintaining a fruit wall was a labour-intensive work that required a lot of craftsmanship in pruning, thinning, removing leaves, etcetera. The extension of the railways favoured the import of produce from the south, which was less labour-intensive and thus cheaper to produce. Artificially heated glasshouses could also produce similar or larger yields with much less skilled labour involved.
This should not be surprising, since if fruit-walls were strictly superior, why were they ever obsolesced? In general, when reading sustainability fetish material like this, you should remember that these sorts of things are like code-golfing or min-maxing: schemes which place unrealistic value on just one dimension, and which can be highly interesting in how one maximizes that dimension, but are only rarely optimal under a more realistic set of tradeoffs. In this case, the fetish is for energy use while running, and mostly ignoring capital, repair, space, and labor costs.