An oscillator needs to have 360 degrees of phase shift to work. The inverter provides 180 degrees, and the crystal adds another 90 degrees. It is the last 90 that is the problem.

In a 12 MHz oscillator, 90 degrees = 20 ns. The propagation delay through the inverter may be 10 ns, and the rc delay due to the inverter output impedance driving the load capacitor can add the balance.

At 32 KHz, 90 degrees = 7.6 MICROseconds. There is no way that you are going to get that sort of delay through an inverter. So you need to build an RC delay. 300K ohms and 30pf equals 9us. The signal amplitudes adjust themselves, which adjusts the delays, and you get the exact 360 degrees.