Risk avoidance with upfront need clarification
Agile in fixed price project fails as there is low scope for any progressive elaboration of requirements. It need to spend time in freezing initial requirements and so it looks like a mini waterfall for product delivery.
Unsustainable development with more pressure on delivery team
In a fixed budget, project delivery takes a peak towards closure of the project. This mounts a pressure over team and process becomes low priority due to aggressive development to deliver the defined milestones. This will burn out both a stable process as well team members capacity. Most of the time it impacts quality of a product. So in a fixed price an unsustainable development becomes a greater challenge for industries.