The lowest possible value is not 1 sat/vb, that's what I intended to highlight before. The lowest is 0 sat/vb, and then 1 satoshi per transaction. The fact that anything less than 1 sat/vb is getting ignored, is because it is negligible already. If 1 sat/vb ever becomes expensive, expect to move move lower than that (e.g., 0.1 sat/vb).

The price of Bitcoin affects the fee rate in dollar terms only due to the non-standard lower limit of the fee rate. Beyond that, the fee rate is primarily determined by competition. Maybe another reason is that, in Bitcoin, a lot of software are configured to set arbitrarily high fees, such as exchanges.