It's always hard to say why something didn't happen, especially when we're talking about something that happened almost 3000 years ago. To my knowledge, there are no surviving texts concerning the rationale behind the evolution of the panoply so we are left with passing mentions of it, archeological finds and, perhaps most importantly, depictions of hoplites in various time periods.

Now, it's important in this context to remember that Greek history is long and spans several time distinct time periods. As with all soldiers, the Hoplite changed with time. It is also important to know that a hoplite armor was never standardized, nor subsidized or supplied by the city states, it was purchased by the individual soldier and quite possibly often inherited. As such there is no standardization to speak of (with the exception of Spartan warriors during part of their history) other than what comes as a result of supply, cultural factors and group adaption.

Not all hoplites fought in the armor you mention. Especially not during the entire time span. It was relatively common for poorer hoplites to wear lighter linnen armor or no armor at all save perhaps a helmet (though the shield, spear and secondary armament were a requirement to stand in a phalanx). Without going into great detail, the full bronze cuirass was used less frequently the further forward we move. Gradually being replaced by the linothorax which was either linen or lether depending on which historians you ask, and finally with either nothing or a tunic or cloak, again, depending on interpretation and historian. If we move even further to the hellenistic period we see a revival of pikemen rather than standard hoplites and heavy armor seems to have enjoyed something of a revival since the pike no longer allowed the warriors to rely on the aspis for protection.

So why didn't they wear more armor? We can only speculate on this topic. Weight was certainly a concern. A full bronze panoply already weighs in at over 30 kilos and on top of that you have a shield weighing in around 6-8 kg and a spear and sword or dagger. You're already packing a lot of weight. Greece is, at least periodically, hot and dry. Marching, fighting and even just waiting around in that kind of armor is hard work. Adding even more weight risks slowing you down and wearing you out quickly. Price is certainly another factor, the more expensive you make the armor, the fewer men will be able to afford it. There is also the skill of the armorer to take into account. Metallurgy was not where it was in the early medieval era or even the roman imperial times, this limits what kind of armor can be crafted.

All of this said, greek history is not my specialty. I'm more of a generalist when it comes to arms and armor and primarily interested in its adaption over time. There is however some guy called Iphikrates who has been known to comment on the topic of Greek history and warfare now and then. I'm sure he will rip me to absolute shreds if I got things wrong here. :)

The amount of armour Greek spearmen carried varied chronologically, geographically etc. but for most of the time a spearman we conventionally call a hoplite had a wide variety of armour at his disposal, including groin, lower and upper arms, thighs, shins, ankles and even toes.

One thing worth noting, however, is that armour was not impenetrable and that even the thickest pieces of armour - helmets, were found with very dramatic holes pierced by various weapons. So everything was a target on Greek battlefield and fatal blow could be dealt anywhere. Choosing how much armour to wear, except availability of workshops and possible funding issues, must've been a compromise between protection and mobility (at least fatigue caused by heat or weight or both).

For more than 250 years (to put this into context, this is the entire ''career'' of now famous Corinthian type of helmet) Greeks mostly relied on their shields and helmets for protection, followed by greaves and, perhaps surprisingly, ankle guards. The popularity of breastplates is uncertain. Bronze cuirasses (which appeared at least from 720's BC) aren't many (and mostly coming from late 7th or 6th centuries), yet artistic evidence shows them as a very common piece of armour throughout Archaic era (cca 750-450 BC). To make matters worse, we have no surviving composite breastplates which could fill the statistical gap here, despite their appearance in both literary and artistic evidence.

Bronze belly guards shaped like half moon, and probably hung from a belt under breastplate (later coming in two parts, hinged) appear in mid 7th century, continuing to late 6th, and outside Greece even longer.

Bronze upper arm guards in various forms appeared sometimes in the first half of the 7th century and remained present throughout the 6th century.

Lower arm guards, similarly appear in early 7th century and continue to the first half of 6th (although artistic evidence continues to the end of Archaic era)

During Archaic era, 6th century in particular, thigh guards were a popular artistic motif, yet the only extant one seems to have come from the first half of the 7th century BC.

Interestingly enough, bronze greaves that were a standard piece of equipment from at least the end of the 8th century,were largely abandoned in Classical age (cca 450-320 BC).

Ankle and foot(including toes) guards, commonly overlooked, are actually quite numerous in archaeological record. They appeared sometimes in the second half 7th century and existed down to the end of Archaic era with some examples of foot guards appearing even in Classical (usually less heavily armoured) era.

Worth noting are some examples, although rare, of reinforced (with metal) footwear which might have been used in military context (bare feet are still dominant nevertheless).

Now, why were all of these largely abandoned in Classical age is a topic worth discussing. I don't think we have a definite answer to that, with several possibilities. The changes in warfare towards somewhat more organised, hoplite exclusive phalanxes, more egalitarian socio-political climate that overshadowed competitive elite from earlier centuries, sheer numbers and supply capabilities etc. might be some of the reasons.