The answer to 'why is my hedge thin grass' is that your hedge may be causing shade or competition that is preventing the grass from growing properly. Hedges can block sunlight and absorb water and nutrients from the soil, leaving the grass underneath thin and sparse.
Some common causes of thin grass under a hedge include: too much shade from the hedge, the hedge's roots competing for water and nutrients, and the hedge's dense foliage preventing adequate air circulation and sunlight from reaching the grass. This can lead to the grass becoming thin, patchy, and unhealthy over time.