Isaiah's explanation (King James version)
Listen to the passage in English
A crackling exegesis by Rabbi Samson Rephael Hirsch*: "For the Shabbat which precedes the anniversary of the destruction of the Temple, the Haphtarah is taken from that first chapter of Isaiah which in broad strokes makes clear to us... the nature and calling of Israel, its place in the midst of mankind, the loftiness of its mission, and the depth to which it has sunk. Its depravity, which made the destruction of the state and of the Temple necessary as the sole means of saving Israel for its everlasting calling. Thereby the mourning of the ninth of Ab is given its sharply defined limits. The Jew does not mourn that thousands of years ago the Temple was destroyed, but that it had to be destroyed. Not over the destruction, but over the causes of the destruction.
For demoralization, not morality, estrangement from, and not approach to, God-fearing was what was effected by Temple visits, praying, and the festival gatherings which were not practiced as means for the true Divine Service which is to be performed in life devoted to duty but as substitutes for it. [v. 11-15] Boundless selfishness, greed for profits, misuse of power in service of their own interests on the part of those in authority and greed for lucre in all classes of the people, the oppression of the defenseless widows and orphans [v.23] was what made the prophet give the people the resounding appellation of "the Lords of Sodom and the people of Gomorrah." [v.10] The people... to whome money and possesions are only to be valued as means of living life int he fulfillemnt of the God-ordained dutied of justice... sunk to the level of Sodom where poverty is considered a crime! Of Sodom, the original pictue of cold, smooth, external respectability under which the most complete selfishness, the utteer lack of consideration for others, the most pitiless harshness of heart...
Therefore every recurring ninth of Ab, is to pose the question to every generation: Is our... present... deeply imbued with the Jewish spirit, so filled with the Jewish way of thinking.. that it could form a worthy enviroment for a Temple of God to be erected in its midst?"
A sweet summary by DovBear: The Temple was destroyed because the leaders of Jerusalem were pious frauds, who used the Temple to justify their selfish behavior.
They were glad to assemble at the Temple for Festivals, Sabbaths and New Moons, to bring fatted rams and the blood of oxen and lambs and he-goats for sacrifice, but none of it meant a thing because (as SRH puts it) they also tolerated "social crimes which undermined the happiness and life of their neighbors."
In the language of the Prophet, they did not "Seek justice, encourage the oppressed, defend the cause of the fatherless, [or] plead the case of the widow." They didn't protect the vulnerable or defend the rights of the innocent. Instead they just kept showing up on the Temple Moun, day after day, with their fat bulls and incense. While vulnarable people went exploited and unprotected, the leaders of Jerusalem gathered on a mountaintop to pay lip service to God. (I'll leave it to you to decide if this goes on today, and to determine if this behavior is more prevelant on the left or the right side of the current political divide.)
Finally, God said "enough," and took the Temple away, not as a punishment, but to prevent its continued misuse as a crutch and a dodge, and to force the people to realize that all prayers and offerings are in vain if the law of the Torah is not kept, that the sweetest odors from sacrifices would not save the altar, the Temple, and the State, if they were permitted to take the place of the rest of God's statutes.
Not incidently, the Hirsch exegesis, coincides with what I wrote last Friday: "God has redeemed us by giving us the power to redeem ourselves -not through hours of prayers, nor through personal stringencies, nor through pilpul, nor through conquering Yesha, but through good works." Or as Isaiah himself puts it in the last verse of the Haphtarah: Zion will be redeemed through justice and by those who return to her with rightuousness.
* Actually, it was Mendel Hirsch, the sage of Frankfurt's oldest son, who wrote this.