I've noticed this as well.
Why do you suppose this is?
ETA: I'm focusing only on those that complain about an inability to do so. AND I'm a preface this by saying that I'm not considering all the many things that may be wrong with said brothers involved in the phenomenon we're all witnessing...
I think its happening for a couple of reasons:
A) The earlier in his career you meet "successful brother" the better (and the easier). Its best to do it while he's still in school (which leads to a whole different set of questions and issues). Because the longer we wait, he's experienced more (with women and what they're willing to do to be with him), the more options he has and accordingly the higher his demands become.
B) The further along in our careers we get, the more difficult it becomes (I think) to turn off the "driven career woman" persona that tends not to function well in relationships. What I mean is that often times, the skills/attitudes needed to ascend the professional world can cause conflict in relationships.
C) Women are forced to make decisions regarding professional versus personal aspirations (career vs. child bearing) and more women nowadays are electing to pursuit certain levels of career success first.
D) We assume that what we've achieved is what they're looking for. And we place more emphasis on our professional and financial achievements than they do, when in reality, the "non-monetary" things become that much more important. Case in point: My SO's best friend is 35 and 6 years out of a divorce. He's decided he's ready to get married again. Dating an attractive woman, a lawyer, who owned real estate (more than 1), had an incredible career and got along well with his friends. When they stopped dating I was like "what happened - she was perfect - she had x, y and z". And his response was "what makes you think that's perfection?".
What we didn't know is that she wasn't interested in doing the things that he wanted to do. He competes in triathalons. He skis and camps. The older he gets, the less difficult finding a financially stable woman becomes so the less emphasis he places on that.
E) Some of us just aren't listening. Certain types of men have told us time and time again what they're looking for - and we argue with it, we call it shallow, we say they're spitting excuses. We can say what we want...but the truth is, they're acting on these ideas that we like to disregard and we're coming up short as a result. Does it mean we should start changing to become what they want? I wouldn't go that far, but I wouldn't say we're clueless as to what they're looking for.
Interesting random observation: I've noticed that the men who seem to most appreciate "equally yoked" women are those who are either married or are in long term relationships. And I wonder - is it that being in a relationship with an equally yoked woman has made him appreciate her being or is that these men (men more likely to marry younger) are more attuned to appreciating equally yoked women...? Not sure.
Just some thoughts.