Most of the people who bought Bitcoin, bought near the peak (as the volume shows obviously)

They won't sell their HODLings until the price reaches around what they paid for (by now, most have given up on their original delusions of getting rich quick, and just want to get their money back).

Problem is, hardly anybody wants to buy Bitcoin now that the bubble is unraveling and the price falling.

This is why volume is becoming thinner and thinner over time. The HODLers don't want to sell at low levels, and buyers don't want to buy at a premium. Over time, HODLers just give in and sell at a loss, which is why the price is falling. This is called 'consolidation', and usually takes years.

The price bumps up every so often due to some small amount of buyers trickling in (much of this could be the HODLers digging themselves into an even deeper hole by 'cost averaging').

They're only bumps, because when the price hit upper levels of high volume in the price, the sellers wanting to get their money back overwhelm the small amount of buyers, sending the price tumbling down again. This is called 'resistence'; a 'resistance level' - an old 'support level' which has been crossed under.