Because the preferreds could have 5x upside to the commons no upside, but not the reverse. To reject the safety moat offered by the preferreds for an imaginary higher upside is foolish, imo.
As far as Ackman, I'd bet he'd prefer the preferreds as well (probably owns a bunch now too), but the math in accumulating them at the time over just buying the heavier traded common shares didn't work for him. Most preferreds trade very sparsely.