They've mentioned this in a few places. One is that the artics are badly needed on the routes where they usually operate, switching them to regular buses would result in extremely poor service around the city, and for many of those routes there are not necessarily useful alternate routes vs. certain subway closures where there are many alternates.
Additionally, many of the subway closures are in environments where the artics would be difficult to maneuver. This is compounded by the sheer number of shuttle buses needed--it can already be hard for regular buses to work their way in and out of their stops, in tight environments like downtown yonge it would be very difficult to handle a ton of artics at every stop.