Sex is determined by genes carried by sex chromosomes (XY or ZW systems), or by environmental cues, especially temperature. Regarding those species that rely on highly specialized chromosomes to determine their sex, why the evolution "deemed to be more appropriate" to develop sex chromosome rather than to rely on autosomes?
They don't have to rely on sex chromosomes. There is considerable diversity.
This matter is extensively considered in the link below:-
http://www.nature.com/ncomms/2016/160704/ncomms12087/full/ncomms12087.html
Given that you already have allels that promote or inhibit the expression of one sex, just imagine two scenarios: the two allels that inhibit the expression of either come together one one autosome pair --> no sex expressed :(
Or: both allels promoting the two sexes come together: either you have an intersex (in animals: how about behaviour and optical signalling, will it be strong enough to attracts potential mates?) or you need to invest a lot of energy to develop both sexual organs (if possible, e.g. not so in mammals, but in plants). Then, you might not have enough energy to grow until reproduction phase. The possible consequences of developing both sexual organs at the same time varies greatly on your study system. Many traits are correlated to the sex expression directly or indirectly.
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