From the various articles, it is not clear that McChrystal has threaten to resign if his request is not denied. As McChrystal was brought it because of his expertise and support of a counter insurgency approach, he might feel unwilling or even not able to lead on an approach, that in his heart, he does not believe in.
The report was leaked after President Obama apparently asked him to delay it 4 to 6 weeks. That time period seems to be what he needs to think through the alternatives and make a decision. Now, in March, Obama moved to this more aggressive counter insurgency approach and he placed McChrystal in charge. Simultaneously, his administration has been evaluating the current state of Afghanistan. From other articles, Obama and his foreign policy team has been meeting - and they appear not to be in agreement. Biden is supposedly favoring the more limited counter terrorism approach.
There clearly is a problem getting these people to appear before Congress. We would be screaming it President Bush were denying legitimate requests for military or administration people to appear before Congress. It also does not seem limited to just that request. At one of last week's excellent SFRC hearings, Senator Lugar said this in his opening statement:
The Committee hearings this week offered the administration an opportunity to explain the challenges and difficult decisions to be made after nearly a year of study. Invitations were issued, but they were declined. Thus we have turned today to key actors and former officials experienced in government, war zones, Afghanistan, and the region, to provide their insight and recommendations. We are grateful that they have accepted our invitation to present timely information to our committee and to all Americans in an extensively covered public forum. I hope that the Administration will soon decide on the time for its views to reach the American people. In any event, it is critical that the full force and voice of the President lead the discussion around this national strategic priority with so many American lives and hundreds of billions of U.S. dollars at stake. Only he can lay the foundation that will gain the confidence of Congress and of our soldiers, development experts, diplomats, and partners.
http://foreign.senate.gov/testimony/2009/LugarStatement090917a.pdfKerry has said that HRC will come to the committee in October - but that will be after the decision is made.
I think it is good that Obama is considering the alternatives - and the obvious thing that changed from March until now is the Afghan election. Someone in one of last weeks hearings (I can't remember which expert)said that a successful counterinsurgency fight depended on a popular, non corrupt government. Any hopes that the election would increase the perception of legitimacy of the government seem to have been dashed. It could have been that Obama wanted to keep this period of possible change in direction private, but the leak of the memo has made that impossible.
(For reasons why he might have wanted - before it became public - this behind closed doors, read this article - which uses JK's haunting question.
http://www.npr.org/blogs/thetwo-way/2009/09/obama_afghanistan_doubts_mean.html#commentBlock ) Even then, he should have pulled in the leaders of relevant Senate and House committees.