The more important reason is that in meeting people cold, as they do in going door to door, missionaries often encountered people - non-Mormons - who were exceptionally well-read and knowledgeable about Mormon beginnings, history, personalities, doctrines, and activities. A number of missionaries came to doubt their religious beliefs as a result of this. A number of missionaries simply quit the church. On the other hand, door to door contact in strangers’ homes also produced an occasional romance, as it must among any religion’s missionaries.
Then there is the internet. There’s something about this that I find sad although I am not a Mormon. The old style teaching was very organized, clear, and linear. You knew where you were when you were being taught, you knew what it was based on, and you could connect all the pieces (as well as they could be connected). The new method, apparently, is to teach “by the spirit”. I have seen this in several churches - the New Apostolic, videos of early Mormon prophets, conferences whose speakers included leaders from Book-of-Mormon believing denominations other than the Utah Mormons, and some charismatic groups. That style is often disconnected and incohesive. I prefer the more linear, better planned presentation.