I've been working with cin input recently, and I discovered that strtol needs a radix: long int strtol (const char* str, char** endptr, int base);, but strtod does not: double strtod (const char* str, char** endptr);. Obviously, double numbers can be represented in other bases, so why this dichotomy?
strtol is usually used with base set to 0 or 10. base==0 causes it to treat the string as a C integer constant. It uses a leading 0 for octal, or a leading 0x or 0X for hexadecimal; otherwise, it's assumed to be decimal. With base==10, it doesn't recognize anything other than decimal (which is more user-friendly; most uses won't expect an input of 010 to be interpreted as 8).
Floating-point numbers can be represented in bases other than decimal, but C++ only supports decimal. (C added support for hexadecimal floating-point in the 1999 ISO C standard, but C++ has not adopted this feature.)
strtod could have had an option to interpret, for example, 1.4 as a base-8 constant, equal to 1.5 decimal -- but there just isn't enough demand to justify it.
Integer input might usefully be written in bases 2, 8, 10, and 16, and strtol permits other bases just because it's easy to do so. Floating-point input is rarely anything other than decimal (and the syntax of a C hexadecimal floating-point constant is unambiguous so there's no need to specify it).