The issue of providing online access to Asterisque is a difficult one. The SMF offers electronic versions of its other journals, and the subscription rates for these has apparently significantly dropped. Asterisque is now the most profitable publication of the SMF and there is a lot of reluctance to join the 21st century and jeopardize this profitability. I agree with Obelisque that the best solution is to buy the old issues, but the ones that are most in demand are also out of print. All of these topics are currently being discussed (electronic versions, reprinting the back issues, ...). It is also not impossible that the Bourbaki seminars will soon be available online, maybe with a five year buffer, as the contract beween Bourbaki and the SMF gives total freedom to Bourbaki.

EDIT : of course, one reason why the standard model for journals does not quite apply to Asterisque is because it's part journal, part book series...