The answer to 'why is my fascia cracked' is that the fascia board, which runs along the edge of your roof, has likely become damaged or deteriorated over time. Common causes include exposure to the elements, water damage, and normal wear and tear. Symptoms of a cracked fascia may include visible cracks, warping, or rotting of the wood.
To fix a cracked fascia, you'll need to inspect the extent of the damage and determine if the board can be repaired or needs to be replaced. Minor cracks may be able to be filled with wood filler and painted, but more significant damage will likely require replacing the entire fascia board. This is a project that is generally best handled by a professional roofer, as they have the proper tools and expertise to ensure the job is done correctly and safely.