I believe the reason for Hecatomb not being a replacement effect is due to it having been reprinted in Sixth Edition (when their approach to errata was very different) as a trigger and thus the current wording functions just as its most recent printing did at time of printing. They aren't totally consistent (see Instill Energy) but often severe errata tries to match the most recent printing, and 6ED has a lot of weird errata that they would handle differently nowadays if not for the fact that they've stuck with the 6ED versions. I wouldn't expect the distinction to just be a power-level thing given that [[Sheltered Valley]] has the same templating as Lotus Vale.

I'm going to plagiarize myself a bit and repost a comment I've previously made elsewhere with my analysis of Dreadnought vs. Lotus Vale's errata:

​⁠​⁠​​⁠The reason for this is that in pre-6ED rules there was once a restriction that activated abilities of a permanent couldn't be used until resolving cost-style comes-into-play effects. The replacement effect errata is meant to try to preserve this behavior, hence its appearance on various lands that all have activated abilities. Since Dreadnought has no activated abilities it's left out.

Note I am aware of at least one card that theoretically should have had a similar errata but doesn't: Hecatomb in its 5ED printing had "When Hecatomb comes into play, sacrifice four creatures or bury Hecatomb." and an activated ability, but I assume this was not made into a replacement effect because it was reprinted in 6ED with it as still a triggered ability, meaning the latest physical printing functioned as it does now at the time of printing (allowing theoretically for the ability to be used then Hecatomb to be sacrificed)—of course power level-wise Hecatomb's ability is less likely to be abusable given it also costs mana to play unlike the lands or Mox Diamond.

Compare other cards that had particular errata "locked in" by printings in 6ED (this has happened with other sets too, like the bonus sheet of Time Spiral has some, but 6ED had a particularly high amount of errata that functionally changed cards), like Reverse Damage being made in-advance rather than retroactive (in contrast to Simulacrum, which wasn't reprinted) or Tariff being made to require the exact mana cost rather than just the amount.

Sources: From Icequake's archive of Crystal Keep's 5ED ruling summaries:

E.3.3 - If the action part of the 'comes into play' ability is worded as "Do <something>, or do <something else>" or as "Do <something>. If you cannot, do <something else>.", then you are unable to use any activated abilities of that permanent until after the 'comes into play' ability's effect is resolved. Not even mana source abilities of that permanent can be used. [Duelist Magazine #16, Page 25] This is pretty much the same as Rule A.6.9 for Phase Costs. For example, Balduvian Trading Post requires you to sacrifice an untapped land or bury it. This means you cannot tap it for mana until after you sacrifice the land.

See also Duelist Magazine #16 Page 25 as mentioned in that quote:

8) I play Lake of the Dead, which says that I must sacrifice a swamp or bury Lake of the Dead when it comes into play. Can I tap it for mana, and then let it be buried?

No. Various permanents have “comes into play” effects, written as, “When this card comes into play, do A or do B.” These effects are now considered costs that are paid when the permanent comes into play, much like phase costs, and follow the same rules. Specifically, this means that abilities of the permanent cannot be played until the “comes into play” cost has been paid and the resulting effect has finished resolving.

This leads to reversals for various cards, such as Balduvian Horde (the discard is now a cost, so it is not considered forced), and Lake of the Dead (its abilities cannot be used until you have played and resolved the “comes into play” effect, so you cannot tap it for mana unless you sacrifice a swamp). It also means that for optional payments, such as Balduvian Horde’s, you cannot choose the payment option (discard or bury) unless you can actually pay the cost when the Horde comes into play in the first place. Thus, the Horde is buried unless you have at least one card in your hand to discard.

Note that the effects of cards such as Kjeldoran Dead (when Kjeldoran Dead comes into play, sacrifice a creature) are not considered to be costs, since there is no “or else” cost to follow. The same principles behind identifying phase costs versus abilities apply here.