I agree with SES. Jesus's point is that they are without excuse because even greater signs have been given to them than to the people of Tyre, Sidon, and Sodom. Each will be judged fairly according to the revelation they had been given.

And SES is right. Tyre and Sidon had been given plenty of revelation, plenty of signs, and plenty of opportunities to repent. So also had Sodom, the main one coming in Genesis 14, especially vv. 17-24. They deserved every ounce of judgment that came against them.
But you're wondering: Is this saying that more could have been done in these towns, and if they had been, someone would have come to salvation? That means there are people in hell who would have turned to God had they been given the chance, making hell unfair. Some are there who wouldn't be there if they had just been given a fair chance.

No, this doesn't mean that. You can never go with the "what if" question: What if that person had been nicer to another—would they have gotten saved? This is an illegitimate line of reasoning.

The point behind the saying is that Korazin and Bethsaida are without excuse (honestly, just like Tyre, Sidon, and Sodom!). It turns out miracles had been done in Sidon (1 Ki. 17.7-24), and Sodom had experienced a godly deliverance (Gn. 14), godly influence (Abraham and Lot), the visitation of two angels and their accompanying miracles that should have been a warning (Gn. 19.11). They have been given a fair chance, and everything required to make a conscious decision had been available.

Jesus is making a qal vachomer argument: "if this, ... then how much more!" The ancients would have understood it this way, and did. It was not, as we Westerners would see it, as "If they had repented, they would not have even been judged!" Instead, they saw it as 1st-century Palestinians would see it: "If the Sidonians and Sodomites deserved their punishment, how much more do you!"

Make no mistake about it, the sins of the people of Sodom were so egregious that they had to have known they were in wrong. Though their consciences should have been enough, but God also warned them and gave them time to repent. Prophecies against Tyre and Sidon (Ezek 26-28) were delivered directly to the people (27:1-3) and the king of Tyre (28:1-2), and they could have repented at any time and still been forgiven (Jer 18:7-8). The city of Ninevah responded to similar prophecies by repenting, and was spared (Jonah 3:4-10)

The people knew they were doing wrong, they were responsible for their actions, but also could have repented of them. But you are trying to say that their failure to repent and believe is not their own fault, but God's fault?!?!?

I've never read it that way; Jesus wasn't speaking to those in Sodom, but to the those in His presence. I think Jesus was being hyperbolic - exaggerating to make a point.

First, Jesus seems to agree with the common Jewish opinion that the inhabitants of Sodom, Gomorrah, Tyre, and Sidon were as completely wicked thus they did deserved every bit of the destruction God brought against them. They were the absolute rejecters of God. If anyone deserved judgment, they did.

Second, Jesus argued that if Sodom, Gomorrah, Tyre, and Sidon were justly condemned by God for rejecting the prophets of the Messiah how much more just is God's coming condemnation against those who reject the Messiah.

In other words the only way the unbelievers could overturn his argument would be to argue that, in fact, maybe Sodom, Gomorrah, Tyre, and Sidon didn't deserve God's judgment—something no one of his day would be willing to argue.

One way to figure out if an interpretation is correct is to see if that is the way those at that time understood it. So, did those at that time go away thinking that Jesus just taught that those is Sodom, Gomorrah, and etc could have been saved if God had put forth a little bit more effort? Or did they go away thinking that Jesus was teaching that He is the Messiah and that they are condemned if they reject Him? Since they killed Him for the latter it is justified to conclude that their understanding was much more in line with my interpretation than yours.

It's likely God saw Sodom as a lost cause, so given over to sin and depravity, that there was no hope of saving it.

He sent two angels to the city, and a mob of men tried to gang rape them. That was the last straw.

Jesus is simply referring to the fact that all that occurred before there was a Savior. God could have sent Christ then, but it wasn't yet time. Capernaum had seen the Savior and still had not repented.

Given Matthew 11:23, why did God have to destroy the city of Sodom? According to this verse, God could have persuaded Sodom to repent by performing miracles in the city, so why did He destroy Sodom instead?