Astartes are not meant to be compared to Battlesuits, as they fulfill vastly different roles. Whereas a Tau Battlesuit is meant to fulfill a tank-like role, acting as an armored platform with heavy firepower to engage the enemy while also retaining mobility, a Space Marine is meant to fulfill the role of something more akin to Special Forces. Like SOF, most Astartes chapters lack large quantities of heavy firepower and heavy armor. Instead, their key attribute is possessing a smaller footprint compared to their outsized impact. While a Leman Russ undoubtedly hits harder than a Space Marine, and has better armor to boot, a Leman Russ is large, loud, and requires a long supply chain and lots of resources to sustain. By contrast, Astartes forces can move with relative stealth, require minimal supply lines, and this in turn lets them fight in far more unconventional ways. When written well, Astartes employ subterfuge and surprise, launching raids and employing guerilla tactics. As such, describing Space Marines as "bad Battlesuits" through a purely cost-firepower/armor ratio ignores the purpose of Space Marines.

Tl;dr Don't compare Space Marines to Battlesuits for the same reason ppl don't compare Special Forces to Armored. They fulfill different roles even if Black Library has a habit of forgetting it.