Mostly the fact that it was still around.

In Old Greece, tyranny was a thing of the past by the time Thucydides was born. Most tyrants we know about ruled in the 7th or 6th centuries, and by the 5th century they had pretty much gone extinct. Sparta had campaigned throughout the second half of the 6th century to oust tyrants all around the Aegean, setting up oligarchies in their place. Athens had driven out its dynasty of Peisistratid tyrants with Spartan help in 510 BC. The cities of Ionia had collectively deposed their tyrants at the beginning of the Ionian Revolt (499-494 BC), and when the Persians put down the revolt, they allowed the reconquered cities to retain their new democratic regimes. By the late 5th century BC, no city-state in mainland Greece was ruled by a tyrant, and most Greeks had developed such an ingrained distaste and fear of tyranny that they designed their entire political systems to prevent a single man ever seizing power again. Thucydides' appraisal of tyranny in "the mother country" fits within a well-established culture of hatred and suspicion of tyrants.

For reasons we don't quite understand, Sicily was different. From the late 6th century onwards, the major Sicilian cities fell under the sway of powerful tyrannical dynasties, and though their people intermittently rebelled and established more popular governments, the tyrants kept coming back. Herodotos relates how the defence of Sicily against the Carthaginians in 480 BC was championed by the tyrant Gelon of Syracuse. His son Hiero achieved a great victory against the Etruscans at Kyme in 474 BC. When Athens fought Syracuse in 415-413 BC, it was run as a democracy, but within a decade Dionysius I, the most successful tyrant of the Classical period, had taken control of the city. Thucydides would already have known about this when he was revising earlier parts of his work. He therefore had to concede that tyranny was not as inward-looking and lacklustre on Sicily as the mainland Greeks liked to stereotype it once they had freed themselves of it.

While /u/Iphikrates gives an interesting response which is definitely worth looking at to answer a different (and interesting) question, I unfortunately think the real answer to the question at hand is that the Landmark Thucydides really isn't a very good translation. I'm a bit too lazy to do the whole passage, but if you look at the Landmark translation (or at least what I have managed to extract from the whole passage from Google magic):

though they would each have their affairs with their immediate neighbours. All this is only true of the mother country, for in Sicily they attained to very great power.

The original being:

ἐπράχθη δὲ οὐδὲν ἀπ᾽ αὐτῶν ἔργον ἀξιόλογον, εἰ μὴ εἴ τι πρὸς περιοίκους τοὺς αὐτῶν ἑκάστοις: οἱ γὰρ ἐν Σικελίᾳ ἐπὶ πλεῖστον ἐχώρησαν δυνάμεως.

"Best" translated a bit like:

[They] accomplished no deed worthy of mention from their own [power], excepting the things relating to their neighbors for each: for [the tyrants] in Sicily attained the acme of power".

There is nothing in Thucydides implying that Sicilians tyrants were unusual, at least in this passage, except perhaps in that Sicily had some rather tricky neighbors.

It is worth noting that I haven't actually looked at this passage before in detail, but I have something of an instinct when finding something weird in the Landmark Thucydides to immediately question whether the translation is wonky, which has served me pretty well. It is a pity because there is a lot to like in the Landmark editions in terms of annotations, maps and other features, but much like Dominoes pizza, the main course isn't really up to the standards of the sides.