Why is one headlight dim

I would check the ballast, especially if this is a HID headlight-equipped car. Those ballasts can and often do go bad after several years, and especially after a new lamp is installed confirms the ballast is failing. That said, usually, if one is going out, the other isn't far behind, so I would do both while you are there. It's generally a when, not if issue with HID headlights.

Without the year of the car we can't help you find parts, so look up your car's model and put HID ballast in the search term.