Here is a BBC analysis article which logically lays out the answer to the title question: why now?
The article is short enough to read quickly, so select the link. :-)
http://news.bbc.co.uk/2/hi/middle_east/6353489.stmUS claims against Iran: why now?
Analysis
By Paul Reynolds
World affairs correspondent, BBC news website
In October 2005, the then British ambassador to Iraq William Patey told reporters in London that Iran had been supplying technology used to kill British troops in Basra.
<snip>
No evidence was produced, other than a suggestion that the Iranian-supported Lebanese group Hezbollah had also used such charges, so the common origin had to be Iran.
US officials have made similar claims over the last year. General George Casey, the then US commander in Iraq, said so in June 2006.
<snip>
So, why now?
If you take the claims at face value, the reason is that only now has the evidence become substantial enough to be made public. The number of attacks is said to have grown as well, so that is another explanation put forward for going public now.
But there are other possibilities as well.
--------- here the article lays out possibilities:
Softening up?
Blaming others
Council deadline
---------- the article concludes with:
The claims
Scepticism