maybe this is more of a question about how many Spartiates were there and what was their actual importance

This is actually exactly what this question is about. The entire reason for this is because of the number and importance of the Spartiates. The Spartiate Similars were the Lacedaemonian state. Without a real political center (Sparta was not a true city, but a collection of politically independent villages organized along an extremely conservative model--like much the Spartans did--and with only the loosest ties joining them) and with an enormous amount of territory to control the ability of the Spartans to maintain their population of citizens was of the utmost importance. It was made even more important by their political constitution, which concentrated an inordinate amount of power into the hands of a very few people--in fact, ultimate political control within all Lacedaemonian territory and in the territory of the Peloponnesian League lay entirely in the hands of the ephors. Think about that for a second. Pretty much the entire Peloponnese lay in the hands of five men. I wrote some writeups about the Spartan constitution here dealing specifically with the power of the ephors and I had another one but like a doofus I didn't save it and now I can't find it

Given all this the importance of the Spartiate Similars as an absolutely loyal body occupying the top of the ladder and capable of forcibly keeping everyone else in their place was crucial. Even more importantly the entire Lacedaemonian system with its absurd social hierarchy would fall to pieces without the presence of full citizens. The Lacedaemonian system was constructed over a period of several centuries, most of them spent in constant warfare with Argos, the most powerful military force in Archaic Greece, against which the Spartans fought a centuries-long series of wars, at first for local dominance but then for survival and dominance over the Peloponnese, to provide full citizens with a set of privileges denied to lesser subjects. Unfortunately, Lacedaemonia had never been a very populous place and the strict social system made full citizens even more precious and rare. Aristotle notes that by his time the Spartiates, both Similars and Inferiors, numbered only one thousand, and during the Peloponnesian War there could not have been many more of them. The Spartiates birth rate was notoriously low due to all the social restrictions placed on them, and Spartiates were disenfranchised left and right for all kinds of silly offenses. The loss of 120 Spartiates was a serious blow

But that's not the only reason. Pylos was the first time such a large number of Spartans had been captured since Archaic times, and it was a battle where the Spartans were completely humiliated, first in their attempt to take the promontory at Pylos with more than ten times the number of Demosthenes' hoplites, then by the pathetic way with which Demosthenes had collapsed their pocket on Sphacteria without allowing then to fight back. Sparta, like the rest of the Greek world, was in shock. Now, this wasn't the first time in living memory the Spartans had been badly beaten. Much of the Peloponnesian War up to this point involving actual Spartiates is a shit show for then, with their forces repeatedly routed by the Athenians. The Athenian marines terrorized the coast of the Peloponnese, Plataea held out for years against an enormously large Spartan army, humiliating them in every attempt to storm the city, and only just before Pylos Demosthenes had annihilated an army led by Similars at Olpae. Lacedaemon had repeatedly sued for peace after these defeats, only to be turned down each time. If those were serious losses in terms of Spartiates then Pylos was nearly apocalyptic. The Lacedaemonian state was in complete chaos after Pylos, with its armies being either defeated or humiliated even in victory everywhere, her land being torn apart, her best leaders either out of date and embarrassed like Archidamus (well, ok, he was dead, but before that) or incapable of fighting like the wounded Brasidas, and now a significant chunk of her most irreplaceable and vital segment of the population in enemy hands. She was very realistically facing extinction within the next few years if the war continued, with a steady stream of her most important resources--her own Spartiate manpower--dwindling away.