5 reasons why test skippers are hesitant to impose follow-on
To avoid pressure in the fourth innings
The cricket pitch starts deteriorating from the very first session in a five-day test match and is at the peak of its deterioration in the last; generally the fourth innings played on an average 4th or 5th Day. The bounce becomes unpredictable, the ball misbehaves and spins the most. With decent bowling attack, even 200 becomes a tricky target to chase in the final innings on a worsening pitch.
Nobody wants to be in such a troubling situation. Even those who opt to bowl first in a test match after winning the toss do not want to be in that situation. Players want to bat on the pitch that is still good for batting and fourth innings is definitely not that time.
Thus, whenever possible, to avoid pressure in the final innings of match, teams do not enforce the follow-on, and went out to bat instead to get the pitch in worst possible condition to bowl the opponent team out.