LeQueen there was a stigma about 11 plus failure, perhaps there still is? The 11 plus was/is a test to determine how intelligent you are. It isn't a good feeling to feel less intelligent that others. As it is this sort of IQ that seems to be valued most by society. You are a superior being, as Plato would see it a leader not a follower. Cognitive scientists have challenged the way we think about intelligence over the last 20 years and perhaps things have changed. The mind is more expandable and intelligence more learnable than we've ever realised.
The 11 plus was designed as a weeding out process to identify those who will benefit the most from higher levels of education. In order to see whom would give the best return on the state's investment intelligence was tested. It was seen, and still is by many, as a unitary quality of mind distinct from memory or perception. Being curious or tenacious were not seen as the same thing.
Your intelligence was seen as a mental competence that would determine how you'd go about a wide range of tasks. Your 'brightness' or 'dimness' would got with you to your meal times to your lessons etc. Being 'dim' stayed with you the same way as being 'bright' did a bit like your hair or eye colour.
Your performance at solving abstract logical puzzles devoid of context, personal relevance and experience - especially if you could do it fast and under pressure was a great indicator of how much 'intelligence' you had. Such tests could be a quick, reliable dipstick of how much intelligence a person had and this became a very important thing to know about them.
The thinking behind the Spens Report of 1938 spawned the 11 plus. Some may still agree with its sentiments including many of the infant teachers I've met with who can just tell so early on who is bright and who is dim which can lead to unconscious labeling and self fulfilling prophecies. As LeQueen said children have a habit of living up or down to our expectations:
Intellectual development during childhood appears to progress as if it were governed by a single central factor, usually known as 'general intelligence'...it is possible at a very early age to predict with accuracy the ultimate level of a child's intellectual powers..It is accordingly evident that different types of children, if justice is to be done to their varying capacities, require types of education varying in important respects.
Some of this is taken from Claxton's research. I am a big fan.