I noticed that most of the depowerable open cell- foil's leader lines are kinda long compare it with LEI leader lines. Is there any explanation on having a longer leader lines.

just a guess, but since foils have the ability to reverse launch... having long leader lines is more comfortable/safe when you reach up and grab on the leaders to reverse.

also, most LEI use a cam-cleat below the bar near the chickenloop for trimming where most foils use a webbing trimmer strap above the bar. thus the leaders neeed to be proportionally longer to equalize with the length of the trimmer.