The clue to this mystery may be that one side died sooner than the other. This could be due to poor drainage and a quick root rot, and then the warmer of sunny side of the pot would wilt first. It could be from overspray of an herbiside being blown by the wind from a neighbor or street maintenance or it could be too much cold wind or direct sun on one side of container. When you pull up dead plants is the soil damp? Are the roots rotted? This could mean poor drainage. Do not let the pot sit in drainage water.  This plant hates to have wet leaves and it rots easily in warm, damp soil. Keep the plant cool and out of direct sun. You did not mention if you are growing this beautiful purple foliage plant as a houseplant or outdoors. It is a tropical annual so once it flowers you can expect it to decline as it is not a long lived plant. The flowers do not have a pleasant odor so most people cut them off quickly before the buds open. The best way to keep it growing is to take cuttings every year and root them in perlite or sand with good drainage. Such an unusual plant is worth the extra effort. Keep growing, Marianne Binetti