>>722974843
because the biggest problem with firstborn marines is that their data sheets had trouble competing with xenos and chaos, in the past when marines became meta it was always because of some gimmick or blood angels (and that one time azrael would solo lead a parking lot) carrying the entire faction
bolters don't actually do a lot of damage, their profiles mean they fire 2-3 shots, and they're really only effective against low-tier infantry, and it meant that space marines had limited profiles when compared to other armies
primaris marines actually have a competitive data sheets now, especially on vehicles because gatling guns, assault guns, and stubbers just put a lot of shots down range
if we look at the current space marine MBT, the gladiator lancer, it has 4 different weapons, and can put out about 11 shots, with 2 of those being str 14 las shots
compare this to the predator which had 3 profiles and put out 7 shots, 3 of those being las shots
those extra low str low ap shots are important for eating up infantry that your opponent is looking to throw away and tie up your tanks in melee combat
basically, space marines lack a special focus (outside of the codex divergent chapters) so they need their vehicles to be tactically efficient because space marines aren't good at being fast/defensive/shooting/melee they are just sort of average at these things
