diurnal tides: why?

You know, the diagram to explain tides is the earth turning inside the hydrosphere that's an ellipse from tidal strain. That gives two high and two low tides per full rotation, semidiurnal tides.

Kayaking class today pointed out that the East Coast gets that, but some places get diurnal tides, one high and one low per day. (And we, for example, get a complicated tangle, but that's for later.) Where does that low-frequency 1/day component come from?

Wikipedia is not satisfying. (It does note that the peak resonance of ocean slosh is at about 1/(30 hours), which is interesting, but not to the purpose here.)

This page notes that

The dynamical theory of Laplace and Airy considered the tides as the effect of the excitation of normal modes of oscillation of the ocean's surface. This explained the phase problem, as well as the extremely various nature of the tidal signatures. Although this was a complete and satisfactory explanation, it is very difficult to calculate tides a priori because of the complex nature of the oceans.

Nod nod nod that's elegant... wait, but what's the excitation? Isn't the excitation something handwavily related to the sinusoid of frequency 2/day that comes from the tidal-strain ellipse? Where's that huge (and sometimes quite clean) subharmonic coming from? It doesn't sound from this description as if Laplace and Airy were invoking nonlinearity.