Biblical reasons: Hiding God? If there is a firmament encompassing the flat earth, the current powers to be, satanist, do not want the masses to know about it or find it. It seems they are protecting the edge by preventing anyone to go there and they are protecting the dome through NASA to prevent anyone from going there too.
Listen to Eric Dubay explain:
Secular reasons: For money and power. For 500 years the powers to be have promoted and protected the model of a round globe. In times past, kings could not get anyone to search for and discover new lands if the sailors were afraid of falling of the edge of the earth. Remember back then they only had sailing ships. Another reason is to protect the institutions now in place like the sciences taught by universities, governments and businesses. If exposed, there would be a wave of law suits against all of them.
Listen to Mark Sargent explain his views on this. If you have time, the whole interview is very good. He answers the why question at around the 43 minute mark and lasts about 10 minutes: