A basil plant becomes droopy mainly due to a few key reasons related to watering, soil conditions, and environmental stress:

• Dry soil is the most common cause; basil needs consistently moist soil. If the soil dries out, the plant wilts quickly.
• Overwatering or poor drainage can cause root rot, which leads to drooping and yellowing leaves. Basil dislikes sitting in soggy or boggy soil.
• Sudden changes in temperature or environment, like heat waves, cold snaps, or transplant shock, can stress the plant and cause wilting.
• Inadequate pruning can also contribute to a droopy appearance as the plant grows leggy.
• Pests or fungal diseases like fusarium wilt can cause wilting and plant decline.

In summary, a droopy basil plant is often a sign of moisture stress—either too little or too much water—and environmental or disease-related stress that affects root health and overall plant vitality.