Why is a temporary name given to an element with an atomic number above 100?
The systematic element names refer to a temporary name and symbol for hypothetical and newly synthesized elements. Because of naming controversies in the past, these names were introduced by the IUPAC to discuss the elements without ambiguity. After the discovery is confirmed, they receive a permanent name.

The IUPAC states in the red book this controversy as follows:


  In the past, some elements were given two names because two groups claimed to have discovered them. To avoid such confusion it was decided in 1947 that after the existence of a new element had been proved beyond reasonable doubt, discoverers had the right to suggest a name to IUPAC, but that only the Commission on Nomenclature of Inorganic Chemistry (CNIC) could make a recommendation to the IUPAC Council to make the final decision. 


As this text suggests, this is a highly political process, that can be dragged out quite easily. The systematic nomenclature and symbols for new elements are stated in IR-3.1.1 as follows:


  Newly discovered elements may be referred to in the scientific literature but until they have received permanent names and symbols from IUPAC, temporary designators are required. Such elements may be referred to by their atomic numbers, as in ‘element 120’ for example, but IUPAC has approved a systematic nomenclature and series of three-letter symbols (see Table II).2 The name is derived directly from the atomic number of the element using the following numerical roots:
  0 = nil
  1 = un
  2 = bi
  3 = tri
  4 = quad
  5 = pent
  6 = hex
  7 = sept
  8 = oct
  9 = enn
  The roots are put together in the order of the digits which make up the atomic number and terminated by ‘ium’ to spell out the name. The final ‘n’ of ‘enn’ is elided when it occurs before ‘nil’, and the final ‘i’ of ‘bi’ and of ‘tri’ when it occurs before ‘ium’.
  The symbol for the element is composed of the initial letters of the numerical roots which make up the name.
  Example:
  1. element 113 = ununtrium, symbol Uut


As for the transfermium elements, their names have been finalised only in 1994 (Pure Appl. Chem., 1994, Vol. 66, No. 12, pp. 2419-2421).

However, for Mendelevium the first publication already suggested the name and a symbol. It was in use before officially recommended by the IUPAC (A. Ghiorso, B. G. Harvey, G. R. Choppin, S. G. Thompson, and G. T. Seaborg, Phys. Rev. 1955, 98, 1518).