I noticed on Windows and Linux x86, float is a 4-byte type, double is 8, but long double is 12 and 16 on x86 and x86_64 respectively. C99 is supposed to be breaking such barriers with the specific integral sizes.
The initial technological limitation appears to be due to the x86 processor not being able to handle more than 80-bit floating point operations (plus 2 bytes to round it up) but why the inconsistency in the standard compared to int types? Why don't they go at least to 80-bit standardization?
The C language doesn't specify the implementation of various types, so that it can be efficiently implemented on as wide a variety of hardware as possible.
This extends to the integer types too - the C standard integral types have minimum ranges (eg. signed char is -127 to 127, short and int are both -32,767 to 32,767, long is -2,147,483,647 to 2,147,483,647, and long long is -9,223,372,036,854,775,807 to 9,223,372,036,854,775,807). For almost all purposes, this is all that the programmer needs to know.
C99 does provide "fixed-width" integer types, like int32_t - but these are optional - if the implementation can't provide such a type efficiently, it doesn't have to provide it.
For floating point types, there are equivalent limits (eg double must have at least 10 decimal digits worth of precision).