Why can't variable names start with numbers?

Well think about this:

int 2d = 42;
double a = 2d;

What is a? 2.0? or 42?

Hint, if you don't get it, d after a number means the number before it is a double literal

Because then a string of digits would be a valid identifier as well as a valid number.

int 17 = 497;
int 42 = 6 * 9;
String 1111 = "Totally text";

It's a convention now, but it started out as a technical requirement.

In the old days, parsers of languages such as FORTRAN or BASIC did not require the uses of spaces. So, basically, the following are identical:

10 V1=100
20 PRINT V1

and

10V1=100
20PRINTV1

Now suppose that numeral prefixes were allowed. How would you interpret this?

101V=100

10 1V = 100

or as

101 V = 100

or as

1 01V = 100

So, this was made illegal.

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