Why is there consistent seasonal variability in the quality of anime?
I was going to agree with you jogging off recent memory however taking a look at upcoming anime for the rest of the year I can't say I agree one season is stronger than another. Here are a few examples for the rest of the year: Akame ga Kill!, Hanamonogatari, Space Dandy 2, Sword Art Online, Fate Stay/Night remake, Psycho Pass 2, Durarara 2, Evangelion 4.0, Log Horizon 2, new Gundam series. There are plenty of other great looking titles too though I can't say I know them well enough to know whether they will be successful or not.

I feel like this is all just a subject of opinion really, though this does seem to be the season for romance anime however, cant recall there being this much of it before.

Last winter did seem to have an unusual amount of mediocre anime though. I only watched a few things last season (winter) other than ongoing shows that were still running from the previous season.