Irons are the main type of account requiring additional bank slots, since many niche items acquired along an iron account's journey and almost all skilling items need to be stored. Raw GP, however, is more difficult to obtain on an iron account than it is on a main account, since GP must be earned directly through coin drops, selling items to shops, and high alchemy. GP on an ironman has a few very tangible uses, namely buying runes and other supplies, but bank slots quickly become astronomically expensive and don't feel like a justifiable use of raw GP past the first few tiers.
If irons could use Death's Coffer money to buy bank slots, it would make the higher tier of bank slots more feasible to acquire while still sinking items/gp from the economy.
holy i never knew that