Why does everyone fly Lufthansa to connect to cities in europe?

I work for an airline in the United States and was wondering and handle baggage and notice that all the pengers that fly with my airline connect with Lufthansa from another city in the USA and then to their main hub in Frankfurt or Munich and then to their final destination within europe is this because Lufthansa is europes biggest international airline with more destinations than any other in europe or because Lufthansa has the best airfares to offer I always wanted to know but am too lazy to research cheers?