I think it is bacause the designers of the language have followed this convention.
I have searched on Google and found that while naming a variable this is a rule which is followed while naming a variable.
Some links are given below:-
The following rules apply to variable names:
- Variable names cannot contain spaces.
- C Programming/Variables
- Variable names by IBM
- Java Variable Naming convention
Variable names are case-sensitive. A variable's name can be any legal
identifier — an unlimited-length sequence of Unicode letters and
digits, beginning with a letter, the dollar sign "$", or the
underscore character "". The convention, however, is to always begin
your variable names with a letter, not "$" or "". Additionally, the
dollar sign character, by convention, is never used at all. You may
find some situations where auto-generated names will contain the
dollar sign, but your variable names should always avoid using it. A
similar convention exists for the underscore character; while it's
technically legal to begin your variable's name with "_", this
practice is discouraged. White space is not permitted.
- Wiki for Naming Convention
In all of the above links you will find that the designers have followed this naming convention for naming the variable.
Also check Is there any language that allows spaces in its variable names
