Probably because it is nearly impossible to stop Hamas from diverting cement to build tunnels.
From the NYT:
Momentum for the rebuilding of the Gaza Strip advanced on Tuesday, with a senior United Nations diplomat briefing the Security Council on a temporary deal between Israeli and Palestinian officials to import cement and other building materials.But there never was a limitation on the number of trucks going through Kerem Shalom.
The diplomat, Robert H. Serry, the special envoy for the Middle East peace process, told the Council that he hoped the deal would lead to a broader agreement on opening border crossings to Gaza and on ending severe restrictions on imports to the Palestinian territory, where the economy was stagnating before the 50-day war this summer.
The Palestinian Authority, led by President Mahmoud Abbas, will have “a lead role in the reconstruction effort,” while United Nations monitors will ensure that reconstruction materials are not “diverted from their entirely civilian purpose,” Mr. Serry said.
Israel has long insisted that its restrictions on a range of goods, including cement, are necessary to prevent Hamas from using them to build underground tunnels into Israel. The limitations are a source of intense frustration for Gazans.
“Arriving at this agreement has not been without its challenges,” Mr. Serry said, according to a prepared statement. “We consider this temporary mechanism, which must get up and running without delay, as an important step toward the objective of lifting all remaining closures, and a signal of hope to the people of Gaza.”
The three-way agreement on reconstruction is between Israel, the Palestinian Authority and the United Nations. Mr. Abbas announced the agreement last Thursday in a televised meeting of the Palestinian leadership. The estimated reconstruction cost is about $7 billion, Palestinian leaders have said. An international donor conference is scheduled for next month.
Donors, however, are likely to be wary of committing money without assurances of a more enduring peace deal.
A further complication is the deteriorating relationship between Hamas and Mr. Abbas’s Fatah faction, which signed a reconciliation deal in April after a seven-year schism. It is unclear whether Hamas will continue to participate in a unified Palestinian delegation for the Cairo talks, which are supposed to resume soon to address unresolved issues in the cease-fire pact.
The reconstruction arrangement would give Mr. Abbas a foothold in Gaza. Hamas, buoyed in public opinion by the fighting, would have difficulty blocking any reconstruction effort, but may limit Mr. Abbas’s operations.
...But it was unclear exactly how the new mechanism would work, when it would begin, or how much material would be allowed through. Moshe Yaalon, Israel’s defense minister, told Israeli military reporters earlier on Tuesday that the number of trucks allowed through Israel’s Kerem Shalom crossing would increase to 380 a day from an average of 250, but that includes commercial goods and food.
Which UN group would monitor the cement deliveries? Are they already Hamas apparatchiks, like UNRWA employees are?
And is Egypt obligated to supply anything to Gaza?
The devil is in the details, and in this case, there is no way that Hamas will not get access to the cement as long as it controls Gaza.