Because they are shite!!!!!!
I went today for maternity bras, i was measured after waiting 30 minutes and woman said I was 32D, that didnt fit, then she tried a 34dd, no good, then she decided I might be an E cup but said she didnt have any maternity in an E cup!
So I trotted off to Bravissimo, had never been before and wasnt swayed by useless marketing twaddle on the wall about how women have never felt so good shopping for underwear blah blah.
No measuring, she looked at my boobs, told me I was wearing the wrong size pre-pregnancy (also measured by M&S), she came in with a bra, put it on, told me to bend over and jiggle my boobs and stood up ...HEP PRESTO the most perfect fitted bra Ive ever worn in my life.
Yes it was more expesive, but my boobs look good and Im comfortable.
I will go back after pregnancy and kit myself out with proper fitting bras.
I left being part of the marketing twaddle highly recommended