Why should resin artists focus on customization?

Customized items as a general rule can charge higher prices. People are willing to pay more because:

1. It's personalized to them. 
2. It's unique
3. They have an emotional attachment to the final product

Are you selling your keychain as a keychain, or are you promoting it as art?

Also if you customize, you can save on inventory holding costs. Making 30 keychains costs you money. And customers have a lot attachment to it, unless it's something like 

1. Their favorite colour
2. Their favorite theme - Beach, Space etc
3. Their profession
4. Their personality - Bike, Car, Music etc.

Otherwise there is no emotional attachment, and therefore they value your product as a commodity.