Machines are simply better at space travel then humans.
There is nothing that humans can do in space that machines can't do better. Machines are cheaper than humans. Machines don't get hungry, or tired, or lonely, or sick. Machines can withstand harsher environments and they don't need a costly life support system.
And machines don't need a round trip ticket.
This trend isn't new. Machines have been taking over human activities for hundreds of years. When is the last time that a book was written by hand (rather than printed by machine)? I don't suppose anyone here is worried about this.
This isn't even a difficult question. Machines are superior to humans in space. There is really no reason to send humans at this point... and there won't be until the time we are ready to colonize.