Since Sinnoh is on a lot of people's minds right now for obvious reasons, I wanna discuss one of its most infamous qualities that is in itself a mystery.
The Diamond and Pearl dex. We all know it, and we all hate it. I think every Pokemon fan who actively participates in Gen 4 discourse has at some point seen or made a joke about the 2 Fire types, Flint's team or something along those lines. What is discussed far less often is
why it was in the state it was in, and for such a well-known issue I don't blame anyone cuz thinking about it no matter for how long doesn't seem to have it ever make any more sense. After bringing up this topic a few times, seeing people's opinions and speculating on my own I've compiled 3 viable-ish and/or commonly circulated theories with varying degrees of issues. Let's take a look, starting with...
Theory 1: Unforeseen Development Complications
This is a theory that I personally came up with in the wake of the DP beta leak that happened a while back. That leak presented a game that looked
incredibly rough with unfinished sprites, low-quality textures and unfinished features galore, especially considering that it was dated merely 5 months before launch, which in turn strongly hints at a troubled development. In addition, there is a piece of circumstantial evidence that suggests the omission of many of the Sinnoh cross-gen evolutions was one made decently late in development, at the very least after the conceptual stages: In Platinum, the duo of Flint and Volkner use Magmortar and Electivire as their signature Pokemon respectively. On top of this, they both use the respective Kanto eeveelutions of their types, and wouldn't ya know it, among the missing evolutions are Eeveelutions. Considering their firmly established friendship dates back to DP it's incredibly likely these Pokemon choices were made to reflect this pairing of theirs, as in it may have been intended to be a thing from the start until the decision was made to lock away these vital Pokemon to the postgame.
The evidence for this theory is solid enough at a glance, but it raises a major question: Why did this never happen again before or after? The former especially is important to consider: Pokemon before this point was no stranger to development problems even beyond the extra notorious Gen 1 and 2 cycles, for Ruby and Sapphire, the very previous games, had such a strenuous, high-stakes development cycle that
it resulted in Masuda himself being hospitalized, and even THOSE were capable of fitting all 135 new Pokemon they introduced with the exception of deliberately rare and elusive ones like Beldum and the legendaries into the main game. It's not like they ran out of time to implement the cross-gen evos in DP either: They're fully existent and playable, just not catchable in the main story. This theory is predicated on the assumption that DP's development in particular was just so utterly nightmarish that they were somehow either unable to correctly implement the encounter tables or when they tried they ran into some sort of bizarre glitch or other difficulty they had no time to resolve and so decided to scrap it, which just sounds like a major stretch to me.
Theory 2: Dual-Slot Shilling
This seems to be the most "popular" theory as far as I can tell, but also is in my opinion the most flawed. For the uninformed, Dual-Slot Mode was a feature in DPPt where by putting a GBA cartridge for one of the gen 3 games in the DS' GBA cart slot you could catch special Pokemon not obtainable in normal gameplay depending on what cart you put in. The theory goes that in order to entice this feature's usage the cross-gen evos were blocked off to be only accessible by using it. Again, at face value, this makes sense and sounds exactly like the brand of stupid jank you'd expect Game Freak to do. Then you look at the actual list of
dual-slot only Pokemon and realize only three lines with evolutions, Magby, Elekid and Gligar are actually obtainable this way. Even if you count Tangela, the only line that requires transfer from an older title to be obtained in DP, that still leaves the vast majority unaccounted for, including the likes of Probopass, Gallade, Gliscor, Porygon-Z, Glaceon and Leafeon, Togekiss and more.
Theory 3: Ill-Conceived Kanto Homage
One curious thing that some have noted about the DP dex is that its Pokemon counts adds up to 151 with the Mythical Manaphy being the last one. This makes it identical in terms of Pokemon count to the original Kanto dex, right down to the last Pokemon listed being a Mythical. This leads some to believe that this was done as some very poorly-thought-out reference to the first generation. Honestly out of all the theories here this is the least worst just because there is nothing directly disproving it. That said, I still find it really hard to believe because if they were really trying that hard to force this homage, then why didn't they prioritize the new Pokemon and get those all in first? With the evolutions included as well as any Sinnoh Mythical/Legend not in the Platinum dex excluded you have 98 new Pokemon, and when you add in the 32 evolutionary relatives to the new evos the final tally comes out to 130. A tight squeeze no doubt, but it's doable with 21 slots to spare as well, and the nature of the cross-gen evos combined with these slots would've ensured decent enough old gen representation.
So yeah, in the end none of these theories are really 100% satisfactory on their own. It could be a mix of some of these factors, perhaps. But what do you think? Got any alternate theories?