From http://www.ecma-international.org/ecma-262/5.1/#sec-15.1.2.2
Step 8a:
If R < 2 or R > 36, then return NaN.
That’s why.
It’s just a arbitrary rule to simplify implementation, probably.
Edit: see comment for the probable reason.
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solved How come parseInt wont convert to base 60? [closed]