Why is Cysteine and Tyrosine used to calculate a sequence isoelectric point?
Every amino acid has a different isoelectric point: a pH where they do not carry electric charge. This isoelectric point depends on the side chains:


By glycine the side chain $-H$ is neutral (while the amino and carbonic acid groups are not) so the IEP is 5.97.
By lysine the side chain $-(CH_2)_4\mbox{-}NH_2$ is alkaline $[R\mbox-NH_2 + H^+ \rightleftharpoons R\mbox-NH_3^+]$, so it will have a more alkaline IEP 9.74 than glycine.
By glutamic acid the side chain $-(CH_2)_2\mbox -COOH$ is acidic $[R\mbox -COOH \rightleftharpoons R\mbox -COO^- + H^+]$, so it will have a more acidic IEP 3.22 than glycine.


Every side chain has more or less contribution to the IEP of the amino acid. In your question


the side chain of cysteine $-CH_2\mbox-SH$ is a weak acid $[R\mbox-SH \rightleftharpoons R\mbox-S^- + H^+]$ so the IEP is 5.07
the side chain of tyrosine $-CH_2\mbox-Ph\mbox-{\scriptsize (p)}OH$ is an even weaker acid $[R\mbox-OH \rightleftharpoons R\mbox-O^- + H^+]$ so the IEP is 5.66


And every amino acid has more or less contribution to the IEP of a protein.

References:


Table of pKa and pI values
Amino Acids with Acidity Values
Introduction to Proteins; Amino Acids, the Building Blocks of Proteins
Electrospray Mass Spectrometry