Why are so many apple crosses mediocre?
I am recently learning about plant breeding. I now know the very basics, but I can't find the answers to some more advanced questions I have. I am considering breeding apples and I am perplexed by the standard line I have heard: you need to make hundreds of crosses to get a decent fruit, and most F1's will be verrry mediocre: small, tasteless, etc. What I don't understand is why the children won't simply be a combination of the parents traits plus a bit of variation for some new recessives expressed. It sounds like the quality declines markedly just because it is a cross. I have heard that other plants also have this property, but some fruits don't have it, e.g. peaches which give a reasonable F1 much of the time.
Related to this I also don't understand why inbreeding depression can occur in some crops, e.g. corn. Well, I read that it happens but don't understand the genetic basis. It seems like there is a Darwinian force behind it, namely the diversity of a land race: the presence of inbreeding depression will keep the diversity of a patch higher, enough to better survive adversity.