I agree that door-to-door is one of the least (if not the least) efficient way of attracting new members. But attracting new members is really only an ancillary goal of the door-to-door work. As others point out, the primary purposes of the work are (1) keeping the Witnesses occupied, (2) reinforcing the exclusivity of the JW movement ("we're the only ones preaching worldwide, thus we must be the true religion"), and (3) providing a distribution channel for the WT Corporation's literature.
And, as others point out, there is little evidence that the First Century Christians preached door-to-door. Perhaps some did, but it is not clear from the New Testament. There is not one single example in the NT of any Christian preaching door-to-door. There are lots of examples of Christians speaking in public places (Sermon on the Mount, Paul's speech at Mars Hill) and informally (Phillip to the Ethiopian, Jesus to the woman at the well and many others).
But the only examples the WT can find are two statements the book of Acts that mention disciples preaching in homes. (And one of those, Acts 20:20, is simply Paul recalling having spoken to certain ones in their homes in the past.) Whether or not these two verses refer to a house-to-house preaching work is unclear. But what is clear is that there are no recorded examples of house-to-house preaching in the Scriptures. If it was so critical, then why did God not see fit to include any examples of it? Why must we rely on two ambiguous verses in Acts?