I already discussed why Harper wants an election as soon as possible. So why hasn’t he already called it?
Canada now has stricter campaign spending laws than ever before; each party gets a small amount of money per vote per year that Parliament sits for use during the campaign, and there are caps on the amounts individuals and organizations can give to a party. There are also limits on how much each party can spend, which is proportional to the number of ridings the party is contesting. So, the Bloc has a far lower limit than the NDP, because they only need to reach people in Quebec, and all the large parties have a strong incentive to run candidates even in ridings they pretty much know they’re not going to win, since they’re permitted to spend a portion of the money allocated for a riding on national advertising. (And of course the Conservatives have been accused of playing hard and fast with these rules, which is the subject of an on-going lawsuit.)
This is all great, but it gives the Conservatives a strong incentive to hold off a little, so that any ads they air now will not be counted as part of what they spend during the official campaign. The rules say they can wait as long as this Sunday to call an election for their assumed preferred date of 14 October, so look for the writ to drop then.