Because it will spark too early to build maximum pressure when spinning at low rpms. Slow idle in this case may be 800 to 1000 rpm. A lean mixture that burns slower will work well with that initial BUT only when there's no load other friction.
This is why its dropping 3 to 400 rpm when going into gear.
An engine heavily loaded needs a somewhat rich mixture and timing to match.
In this respect the engine needs at idle are similar to the engine needs for near max power for other purposes.
Josh
In terms of sequence, first need to get the transfer slots set to the middle of their working range. .020 to .030 on the primary maybe a bit less on the secondary. With these newer 4 corner idle carbs often the transfer slots are a bit wider or longer than older Holleys, but even those Holley said no more than .040" exposed below the plate. Keep notes so you'll know the relation of thurning the idle speed screw to the amount the slots open. Otherwise you'll have to take the carb off each time you adjsut it. Thats nuts.
Go to the B_G ballpark initial timing suggestion chart posted in the howto section here. That will give you a range of initial idle timing for a starting point. Details depend on when the intake valve closes, overlap, crank arm position so what works out as best in the end varies with these and other factors that put heat into the combustion chamber at idle. Once you get strong idle, then you can fool with a short timing curve. Do it right (like DC/MP did) and there's some 'free' power at the top.