Tsunami's generally involve the whole water column as part of the wave. The normal waves you see are surface waves, and as the name inplies are only on top of the water column.

The wavelength of a tsunamis is MUCH greater than that of a surface wave. In open water the wavelength of the tsunami can be hundreds of km while a surface wave has a wavelength of a few 10's of meters.

Instead of stacking up, a tsunami "stacks long". It may be a short wave (height wise), but it keeps coming and coming and coming because it has such a long wavelength.

As everyone else says, a tall wall of water is much more photogenic/cinematic and scares people--probably because people have plenty of experience with surface wave with very few having experience with an actual tsunami.