Why has Barack Hussein Obama failed to keep his own pledge to finance his presidential campaign with public funds?

In February, this is what the two candidates had to say on this subject:

Obama said "Senator John McCain has already pledged to accept this fundraising pledge. If I am the Democratic nominee, I will aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election."

McCain said "I made the commitment to the American people that if I were the nominee of my party, I would accept public financing," McCain said Friday in Oshkosh, Wis. "I expect Senator Obama to keep his word to the American people as well. This is all about a commitment that we made to the American people. I am going to keep my commitment," he said. "The American people have every reason to expect him to keep his commitment."


NOW, Obama says that he rejected private funds before he decided to accept them and will not, although he promised to fund his campaign with public financing, use public financing.

McCain stands by his promise to use public funds.

Why has Obama failed to keep his promise on this issue?