Sinas Chinam The two Batei Mikdash were destroyed because of aveiros, but the aveiros of these two eras weren't the same. The primary aveiros of the first Beis HaMikdash were avodah zarah, giluy arayos, and shefichas damim. The second Beis HaMikdash was destroyed because of sinas chinam.
There was another difference between the aveiros of these two eras. The Gemara (Yoma 9) states that one of the differences is that "the first Beis HaMikdash, their aveiros were revealed, and therefore, the date when the galus would end was revealed to them. During the second Beis HaMikdash, their sins weren't revealed, and therefore, the end of the galus was not revealed."
Rashi explains that by the first Beis HaMikdash, people were transparent; they didn't conceal their true selves. Everyone knew who was a tzaddik and who was a rasha. They were "revealed," and therefore, it was revealed to them that after seventy years, they would return to Eretz Yisrael (see Yirmiyahu 29:10). But by the second Beis HaMikdash, there were resha'im who pretended to be tzaddikim. No one knew who was a true tzaddik and who wasn't. They were concealed. Therefore, the end of the galus was concealed from them, and they weren't told when Moshiach will come.
Rebbe Yonason Eibshitz zy'a (Yaaras Dvash 7) has another explanation for this Gemara. "During the first Beis HaMikdash, their sins were revealed." This means the nation knew the reason they were being sent into exile, and the reason for the Churban. The navi revealed to the nation that they were being punished for avodah zarah, giluy arayos, and shefichus damim (as it states in Navi). The Jewish nation knew what their sins were, so they knew how to improve. Therefore, it didn't take them long to improve their ways. Seventy years later, they returned to Eretz Yisrael.
However, "In the second Beis HaMikdash, their sins weren't revealed." There were no nevi'im during that era to tell them the cause of their punishment. Although the chachamim told them that the destruction was caused by sinas chinam, it was hard for the people to accept that because sinas chinam doesn't seem to be such a severe aveira. They didn't know the reason for their punishment, so it was harder for them to do teshuvah. Therefore, "the end of the galus was not revealed." Without our teshuvah, we are still in galus, after almost two thousand years.
It is still hard for us to recognize the severity of sinas chinam, which is the reason nearly two thousand years have passed, and we haven’t yet been redeemed.
