How has the hash rate for bitcoin continued to increase despite price drops and miners dropping out? Despite price dropping in 2018, the hash rate for bitcoin tripled from around 15 EH/s to 45 EH/s. Why is this the case? I am aware that the network difficulty adjusts meaning that if miners drop off, it will be more profitable for others to mine but what are the key factors driving the hash rate increase? Is it related to the increasing ledger, an increased amount of miners, and/or other factors?
the hash rate for bitcoin tripled from around 15 EH/s to 45 EH/s. Why is this the case?


Well, at the simplest level, that means more hashing power is being used to secure the network. 

This could mean that a larger number of ASIC units are running, and/or that more efficient ASIC units are being created and used. A more efficient ASIC would perform more hashes per second, for a lower cost per hash per second. 

Every miner will have their own costs, motivations, etc, so it’s hard to speak broadly about ‘what miners will do if price is x and the hashrate is y’. For example, a price drop means some miners with the highest costs may no longer be able to operate profitably, but perhaps those miners are ideologically driven, and will take a loss with the hope that the bitcoin they mine will one day be worth more than it is at that time. Etc.