Aluminum needs more heat than steel because it conducts heat away faster. It also has a thin oxide layer that has to be cleaned off before welding, and it doesn’t show heat color the way steel does, so the welder reads the puddle differently.
Aluminum welding usually runs on TIG with high-frequency AC, or on MIG with a spool gun. Stick aluminum exists but it’s rarely the right call. Pick the wrong process or the wrong filler and the weld cracks.