Because Roku is a one-time purchase, where additional cable boxes are monthly fees. I already own the Roku. By taking away HBO Go access on it, I can no longer watch HBO in that room at all.
Last I checked, all the content wasn't available on-demand either. There was a larger selection on HBO Go versus On Demand. OD only has the last 1-2 seasons of the series it has at all, and has fewer movies. HBO Go has all the seasons of all the series.
Is your iPad permanently hooked up to your TV? Many people have rather large HDTVs and would much prefer to watch on there rather than on a tiny screen you have to carry around or dock somewhere.
Last I checked, all the content wasn't available on-demand either. There was a larger selection on HBO Go versus On Demand. OD only has the last 1-2 seasons of the series it has at all, and has fewer movies. HBO Go has all the seasons of all the series.