Why does a protein crystallize with a specific salt but not with another?
It would be a lie to postulate we knew much about the formation of protein crystals. Yes, we know the basic theory — that we need a supersaturated solution in which the protein is not denatured and which would then slowly evaporate until hopefully a single crystal starts growing — but we don’t have the slightest of clues of what exactly helps and what doesn’t help.

This is why protein crystallography is still trial and error (and nothing else). You try thousands of different conditions in an automated manner, select those with promising results for further examination and finally — if and only if you are lucky — end up with crystals suitable for X-ray diffraction.

Therefore, we don’t know and nobody knows why magnesium chloride might help the crystallisation of your protein but sodium chloride, calcium chloride, aluminium chloride or magnesium bromide don’t.
We don’t know and nobody knows why a $\pu{1.3M}$ solution of magnesium chloride might help the crystallisation but a $\pu{1.4M}$ or a $\pu{1.5M}$ solution don’t.
We don’t know and nobody knows why $\pu{23^\circ C}$ might be a good temperature for crystallisation but $\pu{21^\circ C}$ and $\pu{25^\circ C}$ aren’t.
And so on and so forth.