Short answer, overall safety and ease of installation relative to Line Voltage.
In addition to landscape lighting systems, low voltage also runs irrigation, alarm, and security systems. It’s industry standard for many outdoor applications, not just lighting.
Line Voltage (120V) carries more energy, and incidentally more precaution must taken when installing it. The National Electrical Code requires Line Voltage to be buried a minimum of 18” beneath the surface. Burying wire at this depth requires much more labor along with the disruption of your landscaping.
The National Electrical Code requires Low Voltage wiring to be buried at a minimum of 6”. At a depth of only 6”, there is obviously less labor and less disruption of your landscaping. This benefits the Client and the Lighting Contractor.