b/c almost all of the original fixed income focussed investors are long gone and the paper is now owned by fairholme, paulson, american funds et al as superior proxy to the common.... at minimum get same or approx same upside as existing common and no worries re dilution
you are partially correct that if the offer is not attractive enough, they won’t accept. and it’s not gonna happen unless 2/3 of each series owners accept
and lastly, the high coupon fannie S, T and freddie Z can all be called/redeemed at par by 2021 i think,. all of the remaining can be redeemed at par now