EDIT: There has been a grave misunderstanding.
The reason why this decision has been made is due to the adverse affect it will have on the poll system.
To fully understand how this is a problem, you need to know the basics of how the Poll system was designed. (In short: this is the part you don't tl:dr)
The poll system operates to control the flow of card ideas' status based on popular opinion. That is mediated through the 5 poll levels.
The reliquary operates based on Zanz's updating of the game, and, while still part of the Poll system, is not part of what you are about to read, so just exclude Reliquary while you are reading this.
The 4 levels, Smithy, Crucible, Forge, and Armory; operate on a monthly schedule.
13 cards from the Crucible polls are sent to the Forge over a span of one month.
At the end of every month, the Forge advances 4 of its cards to the Armory.
The smithy provides a steady income of new card ideas to the Crucible polls.
The 18 card cutoff rule, which was set in place earlier this year, keeps the flow of cards strictly to the pattern above.
Now let's say that pseudoquanta was allowed into the current poll system as Other cards.
The Other poll will become flooded with new pseudoquanta ideas very quickly, even with the 2 cards per author per week rule on Crucible Candidates, since the Other poll is only reset once every month.
At the rate in which these pseudoquanta cards are being made, the Other poll would quickly break 20 cards, possibly even 25 after the first round of Crucible candidates go through. That will happen 4 times before the Other polls are set back to 18 cards. The poll will have a total count somewhere around the 25-35 range.
With the 18 card cutoff rule in place, only one of those 25-35 cards will move to the Forge. The rest of the cards, non-pseudoquanta cards included, will have a much greater chance of being moved to the archive; no matter how good an idea it may be. This will result in the very unwanted situation of having to send 10+ cards to the archive. This is very bad because those 10+ cards could be any card, that includes pseudoquanta cards as well as the non-pseudoquanta Other cards.
This will have an incredibly negative effect on the morale of the creators and the stress of the Curators. Creators will lose out on points, while the entire situation will create much more work for the Curators; not to say we can't handle it, but the fact remains.
In the worst case scenario, an entire series of pseudoquanta cards may be wiped out in a single cycle of the Other poll. That is completely unfair to the card creator and severely breaks the system.