Best Western doesn't have a franchise structure that fits Accor's, plus they are all over the world, catering mostly to owners of properties that otherwise wouldn't match the requirements of most hotel chains (except for Wyndham or Choice I guess) or for some reason do not want to join one. So the first two things that would happen when one would acquire BW is half of the hotels leaving and the other half would need to be terminated.
Accor is capable of acquiring independent hotels from e.g. BW (this is how a lot of new Mercures come about), plus the Pullman brand was specifically revived to poach older Hiltons and Sheratons whose franchises were about to lapse. In the US, they showed they can work with operators using the current portfolio of brands by opening the Ibis Styles at LaGuardia. This is more about the corporate will to actually engage in the region than anything else. In Central and Eastern Europe there was a corporate deicision to increase coverage and suddenly they were able to add hotels even in places like Bosnia or Slovenia.