A cleric making an attack has to contend with the possibility of missing, and accomplishing nothing for the round, where Cure Wounds is almost guaranteed to be successful.
Without bonus action healing, the cleric will be forced to choose between healing and attacking each round. Generally speaking, healing won't always be worth spending an action, given the cost in resources and the limited throughput. You might only want to heal when it is situationally appropriate, rather than as a low-investment pseudo-obligation whenever you don't have something better to do with your bonus action. That means you get to conserve spell slots, and combat will be over more quickly, which is a win-win situation for everyone.