The difference between [ and [[ is quite fundamental.

• [ is a command. Its arguments are processed just the way any other commands arguments are processed. For example, consider:

  [ -z $name ]
  

  The shell will expand $name and perform both word splitting and filename generation on the result, just as it would for any other command.

  As an example, the following will fail:

  $ name="here and there"
  $ [ -n $name ] && echo not empty
  bash: [: too many arguments
  

  To have this work correctly, quotes are necessary:

  $ [ -n "$name" ] && echo not empty
  not empty
  

• [[ is a shell keyword and its arguments are processed according to special rules. For example, consider:

  [[ -z $name ]]
  

  The shell will expand $name but, unlike any other command, it will perform neither word splitting nor filename generation on the result. For example, the following will succeed despite the spaces embedded in name:

  $ name="here and there"
  $ [[ -n $name ]] && echo not empty
  not empty
  

Summary

[ is a command and is subject to the same rules as all other commands that the shell executes.

Because [[ is a keyword, not a command, however, the shell treats it specially and it operates under very different rules.

That's a limitation of the way in which the shell breaks lines into tokens.

The shell reads lines from the input file and According to section 2 "Shell Introduction" converts them to either a word or an operator:

2. The shell breaks the input into tokens: words and operators

{ is a reserved word

Some words are reserved words

Reserved words are words that have special meaning to the shell. The following words shall be recognized as reserved words:

! { } case do done elif else esac fi for if in then until while

Words, to be recognized as words, must be delimited.

Reserved words are recognized only when they are delimited ...

Mostly by blanks (point 7) and by operators.

7. If the current character is an unquoted <blank>, any token containing the previous character is delimited and the current character shall be discarded.

( is an operator

Operators stand by themselves:

whereas operators are themselves delimiters.

Where "operators" are either:

3.260 Operator

In the shell command language, either a control operator or a redirection operator.

Redirection operators are:

Redirection Operator

In the shell command language, a token that performs a redirection function. It is one of the following symbols:

<     >     >|     <<     >>     <&     >&     <<-     <>

Control operators are:

3.113 Control Operator

In the shell command language, a token that performs a control function. It is one of the following symbols:

&   &&   (   )   ;   ;;   newline   |   ||

Conclusion

So, '(' and ')' are control operators while '{' '}' are reserved words.

And the exact same description of your question is inside the spec:

For instance, '(' and ')' are control operators, so that no <space> is needed in (list). However, '{' and '}' are reserved words in { list;}, so that in this case the leading <space> and <semicolon> are required.

Which exactly explains why an space (or some other delimiter) is required after a {.

This is valid:

{ echo yes;}

As is this:

{(echo yes);}

This:

{(echo yes)}

Or even this:

{>/dev/tty echo yes;}