"Up until now, they have sadly refused to back efforts for negotiations with the Taliban," Karzai told Der Spiegel magazine in comments published on Sunday.
The Bonn meeting will seek to chart a course for Afghanistan after the NATO withdrawal in 2014, but a boycott by Pakistan has dealt a blow to already fragile hopes for a roadmap.
Pakistan is seen as vital to any prospect of stability in the war-ravaged country a decade after US-led forces ousted the Taliban, who had offered safe haven to Al-Qaida leader Osama bin Laden.
But Islamabad pulled out after 24 Pakistani soldiers were killed in cross-border NATO air strikes a week ago, although sources close to the German Foreign Ministry said the Pakistanis would be kept informed of progress at the conference.
The United States has voiced regret over the strikes but has stopped short of issuing an apology, while the American military conducts an investigation.
Islamabad has so far refused to take part in the probe, exacerbating fears of prolonged problems between Pakistan and the United States.
Pakistan, reacting to fury from its people over the attack, shut down NATO's vital supply line into Afghanistan and boycotted the Bonn conference.
US Secretary of State Hillary Clinton called Pakistan's Prime Minister Yousuf Raza Gilani on Saturday to offer condolences over the strike.
Clinton "reiterated America's respect for Pakistan's sovereignty and commitment to working together in pursuit of shared objectives on the basis of mutual interest and mutual respect," in the call, the State Department said.
A statement from Gilani's office said he told Clinton that Pakistan's absence from Bonn was not open to review since it had received the backing of parliament's national security committee.
The committee "has supported the decision of the cabinet not to participate in the Bonn conference," the statement quoted Gilani as saying, adding that parliament was also looking into the broader relationship with Washington.
Pakistan's decision deals a blow to hopes for drawing up a roadmap for Afghanistan's future, 10 years after Germany staged its first international meeting on Afghan political transition following the fall of the Taliban.
As a neighbour with historic ties to the Taliban, Pakistan is considered integral to ending the decade-long conflict, but experts say a boycott matters less now that initially modest expectations for Bonn were dramatically curtailed.
Diplomats had hoped the conference would help broker peace with the Taliban, but the September assassination of Kabul peace envoy Burhanuddin Rabbani derailed those efforts and contacts are said to have achieved little.
Karzai also appealed for aid to his war-ravaged nation to continue after 2014 when the last NATO combat troops are due to pull out, leaving responsibility for security with Afghan forces.
Stressing that Afghanistan would be "more than ever on the frontline," he said: "If we fail in this war, which threatens all of us, it will mean a return to the situation before 9/11."
The Afghan leader conceded that "sadly we have not been able to provide security and stability to all Afghans – this is our greatest failure."
Afghan Foreign Minister Zalmai Rasoul appealed on Saturday for international support for his country after NATO troops withdraw.
"After 2014, we will continue to need long-term support from our friends in the international community," Rasoul said at a discussion forum in Bonn.
His German counterpart Guido Westerwelle vowed at the forum that the world would not abandon Afghanistan.
In an interview in Sunday's Frankfurter Allgemeine Zeitung, Westerwelle again voiced his regret over the Pakistani boycott of the conference, which will gather delegates from 100 nations.
"Pakistan has more to gain from a stable and peaceful Afghanistan than any of its neighbours," he said.