Actually, they have changed to some degree.
Before 1900, Bible Students handed tracts to people as they left their church (ow!)
In order to distribute millions of tracts and there only being 6,000 Bible Students, the WTS had them printed by secular firms and distributed through the newspaper boys.
Then, testimony cards were printed up. A JW would go up to the door and hand the card to the home owner to read.
JWs placed a whole series of books, and delivered them at a later date.
Then there were the phonographs and sound cars and placards.
In 1943, the WTS started have home "model" bible studies. (before that there was no formal study program as today)
In the 1950's JWs delivered 8 scripture sermons at the door.
Then in the 1960's and 1970's there was the six-month bible study program in the Truth book.
Then they switched to 2 books, then back to 1 book, and now back to 2 books.
But it is probably due for a change.