Why do planars have poor bass extension?

I've been wondering about this. Is it because the excursion is limited because of the tension of the diaphragm & low BL limits its excursion?

On a related note, what techniques do companies like Magnepan use to get more bass out of their low-frequency drivers? Do they put bigger magnets in the grid than would be feasible for the midrange? Heavier diaphragms?

Finally if anyone knows a good reference for learning about the TS parameters of planars and if/how such calculations differ from dynamic speakers, that would be awesome!

Thanks all!