The following are some excerpts from an internet search:
--"Some modern MTBs come with a curved downtube. The purpose of this engineering is to create clearance for long travel suspension forks.
When the fork compresses, the front tire gets closer to the frame. If there isn’t enough clearance, the wheel may get in contact with the frame."
--This comment from https://www.mtbr.com/threads/down-tube-shaping-in-last-few-years.790823/ initially refers to the curve of the down tube at the headtube junction but goes on to mention the bottom bracket junction curve. In between are reasons given that apply to both areas of curvature:
"The hydroformed tube shape creates a structural advantage at perhaps the most vulnerable part of the frame. When obstacles are encountered, the bottom of the head tube is driven back into the down tube. The more perpendicular these tubes are to each other at their intersection, the stronger the structure. The tube shape allows for a more perpendicular intersection and for much more of the downtube to absorb the force, instead of it being concentrated in one area (right where it meets the head tube). This eliminates the need for gussets at the down/head tube junction and saves weight.
All the frame manufacturers who use hydroformed tubes are doing that, often with a similar shape at the other end of the downtube where it meets the BB. Makes for a stronger/lighter frame by allowing the entire downtube to absorb the major forces."
--The one explanation I found to be the best and makes the most sense comes from our very own Stack Exchange. Check out: S shaped down tube (MTB).