We're a (relatively) small country, and we play lots of other sportsThe population of England is around 50,000,000. The population of Germany is around 80,000,000. Even without the fact that it's bigger, consider all the sports where we do as well as Germany (or better). Germany are nowhere to be seen in rugby or cricket.
But if you actually look at England's overall performance since 1966, this is certainly not a freakishly bad performance. There's 3 world cups that we didn't qualify for. We didn't get past the 2nd group stage in 1982.
And if you were there in 1990, England really got quite lucky to get where they got. They were unimpressive against Belgium and it looked like penalties until a last-minute goal by Platt. Against Cameroon, my memory was Cameroon played a load of really sloppy fouls and gave the game away.
What I'm trying to say is blame the manager all you like. The fact is that 1966 or 1990 might happen again, but chances are that it was a blip, and assuming we'll do much better than quarter-final is kidding yourself.