My daughter is 2 and she only has her 4 front teeth. What could be causing the delay?

Genetics. Eruption patterns vary widely among youngsters. Have her examined by a pediatric dentist who can determine how far along her other teeth are in erupting.
Delayed dentition. U should take her 2 see a dentist anyway even if she has only 4 teeth. Her x-ray will show if she has delayed dentitio or absent teeth whcih is a rare condition but it will allay ur fears to know she has teeth that just are taking time to erupt.
Slow . Some people develop quickly, some slower, if the teeth are there , they will erupt. Have patience. A visit at the dentist can produce an xray to confirm the presence of the teeth if you want to know now. See tooth chart http://www.Mouthhealthy.Org/en/az-topics/e/eruption-c.