What is significant about the notice about the absence of the temple coming after the mention of the keeping of the feasts and sacrifices?

Ezra 3:6 (ESV)

6 From the first day of the seventh month they began to offer burnt offerings to the LORD. But the foundation of the temple of the LORD was not yet laid.

The people of Israel showed their dedication and commitment to God by their adherence to the law of Moses and their eagerness to do all the sacrifices and feasts that God required of them. However, they did not come back to Jerusalem to build the altar only. The came back to rebuild the temple (Ezra 1:2–5, Ezra 1:7; Ezra 2:68). Thus, even though the altar and the offerings are in place, this is not yet the climactic moment of their return. The temple must still be rebuilt.