The root problem, however, lies in the nature of mathematical teaching: students need to see several consecutive slides at once. This is especially important for formula rich and calculation rich classes like Calculus, ODE, PDE and Complex Variables where there are sequences of large formulae.
Usually a lecture hall has just one screen but even if it has more there is no reasonable mechanism to show different slides on different screens.
Meanwhile any decent lecture hall has at least 2 blackboards (or whiteboards) each with a capacity of the slide, and usually more, up to 6. Sure, we are talking about decent lecture halls, not someting like FE 114a.