Why are there some unoccupied "megaguilds"?

The authors reference the idea of “Class-1” constraints (Schwenk, 1995), which are hard limits on what evolution can produce: they don’t say what will evolve, only what cannot evolve easily because the world (and the organism’s own design and energy budget) makes some directions through phenotypic space impractical or impossible. Phenotypic space = the abstract set of all possible body plans, behaviours and life-history strategies an organism might have.

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The three general rules suggested by the author are…

(1) Either the animal or the food should be mobile.
If food is fixed (e.g., a microbial mat, attached suspension particles), then a successful consumer must either be fixed in the same place and intercept those resources (filterer, absorber) or be able to reach and move across the food (grazers, deposit feeders). Conversely, if prey are motile (small nekton or epifaunal animals), predators must be mobile enough to catch them. 

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(2) The animal and the food should occur in the same tier.
Tiering is vertical separation on the ocean floor (pelagic, erect, surficial, infaunal). Predation, grazing and many feeding modes require spatial overlap. A suspension feeder elevated into the water column cannot efficiently feed on organic matter confined to the sediment surface; an infaunal miner cannot exploit epifaunal biofilms. Different tiers therefore function as ecological barriers between potential interactions.

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(3) Motility can be limited by the physical properties of the medium.
The sediment surface, mudgrounds, and water column impose different mechanical challenges. Some motility modes require particular structural or physiological investments. If the medium is too cohesive or too unconsolidated, certain types of movement are energetically unfeasible or mechanically impossible.