She can't go by Vice President Harris because former presidents retain the title Mr. President. By title, Trump outranks her, not a good optic in debates.
"Harris" is a rather bland name where Kamala is more distinct.
The US Presidency has always been a popularity contest. The most charismatic candidate wins every time with policy coming in a distant second. Democrats lose when they forget this (Hillary, Biden). Kamala is brimming with personality, so she has a real shot - under the name Kamala.