Question

Why did Mendeleev not include krypton in his periodic table of 1871?

Answer 1

Because the initial publication of the Table preceded the discovery of krypton by about 30 years.

Explanation:

Krypton was identified by William Ramsay in 1898. Its very name means the "hidden one". As a Noble Gas it is highly chemically unreactive, and would form very few compounds.

Note that from a Periodic perspective, the late discovery of krypton makes sense. Chemical inertness and volatility (i.e. low boiling point) do not lend themselves to chemical identification of an element. Ramsay was probably a VERY accomplished experimenter (so of course was his team); he was a highly deserving Nobel Laureate.