Because he hasn’t spent that much money NOT to show it.
The claims of multiple millions of dollars spent to fight release come from claiming that all the money paid to a legal firm to wrap up activities from the campaign were spent to fight release simply because that was one of the many things they did. That would be akin to claiming that because (hypothetical situation here) Donald Trump has a lawyer on retainer and paid that lawyer to fight a speeding ticket, everything Trump paid that lawyer for all that he did was actually paid to fight the speeding ticket, nevermind all the other activities the lawyer participated in.
So why did he not release it earlier? Personally I think it was a brilliant political move. Rational analysis showed how ludicrous the birther claims were and by dragging things on, he had the ability to make it appear many of his political opponents who opposed his policies, on what were very rational grounds, were actually associated with birthers and therefore off their rockers.
Yes, polls showed that a good % of the population were following birther theories, but that % of the population was viewed as likely to never consider voting for him anyway, and painting his opponents as more extreme helped with the moderates.
And personally, watching all the republican congressmen refuse to refute birther theories, I think that strategy was working for him. I’m not sure why they’re releasing it now... seems like they’d be better off politically just ignoring birthers...
Then they shouldn’t have released it. They released because of polls, which have worsened, including among people who may have voted for him.