conventions didn't have much to do with it.
The 2nd SW was the same but without all the WMD, partially because the Inner Sphere's warships were rapidly being destroyed and partially because it was recognized by all sides that humanity was literally bombing itself back to the stone age. The 1st Succession War was an all-out conflict with orbital bombardments, nukes, etc. Mercenary forces seem to care more about this than regular troops do in some cases because if they start a fight in an urban area it can hurt their MRB/MRBC rating. unless there's a military objective within a city, in which case the assaulting force is supposed to do all they can to minimize damage to civilian property. The Ares Conventions do prohibit fighting in cities.
By the 31st century the Ares Conventions are just sort of a shorthand for "civilized warfare." The Star League was actually the first to abrogate them in the Reunification War against the periphery states.