Things like growing at high speed with over-watering to promote size. Plus there's the "wuss" factor, whereby focus groups determine that the public prefers a "milder" flavour, so growers favour cultivars with less flavour, because that's what supermarkets demand. Stronger varieties become expensive to produce so far harder to find. Very sad.
Certainly tomatoes are generally flavourless from UK shops. The ones we grow ourselves are nothing special, but are a revelation compared with the very pretty but boring-as-shit ones from Tesco.
Even things like onions and potatoes which are less affected, are still weaker and less distinct from each other. The recipe says to buy a fluffy potato like a Russet or King Edward for mash, what I find is that almost anything will produce the same not-especially-good mash these days, because the growers don't want a highly distinctive product. They want an "all-rounder" with vague characteristics of the variety they're supposed to be producing. That way they sell to a wider market.