Enthusaism for field service has gone down, mainly because of one thing.
This religion is getting farther and farther away from its 'armageddeon around the corner mindset'.
Don't get me wrong, fervor about end times is still a key component to this religion. But you can never get the same kind of excitement out of 4th and 5th generation believers as you would out of new converts who believe that we are on the brink, especially if there are dates and time tables attached to the beliefs (i.e. 1914, 1925, 1975).
The Watchtower always refers back to people who where out in service during the times there was a date in place as "full of zeal". Big suprise there!
While there will continue to be an atmoshpere of anticipation for the end times to be fullfilled, it simply is not going to reach the kinds of peaks that it could if there where more specific claims as to when it is going to occur. Time continues to go on and people become less enthusiastic about giving the great "warning work".
In western lands feild service has evolved mainly into a time filler. Something you do because it's part of your religions rules. Time has slowed down passion for their beliefs.