Because {} are rarely used in standard texts, contrary to () and [], and because texts in lambdatalk are written without any escaping characters like " or '. For instance you can write 1+2 is equal to {+ 1 2} to get 1+2 is equal to 3, something you can't do in other langauges, as far as I know.
alain marty
In 13 I was answering to 5 and 6
I would be happy if somebody could give me his opinion about this work : http://lambdaway.free.fr/lambdaspeech/, beyond the relevance or not of the choice of braces. Thank you.
Alain Marty