I think because they are one of the most frequently reprinted cards combined with them being specialized and low usage (how is it a tournament staple?).
In their current text, they are available in R&S, EM, UF, DS, HP, PK, MT, SW, MD, RR, UD, as well as them already being a league promo back in Nintendo's first year. That's 12 releases--some of them back-to-back. That doesn't even count the ones you need a reference for, like the super attractive Neo Genesis versions. Not to mention they haven't been rare cards since PK (they're common in SW and Uncommon in everything else post-PK) so in those 11 sets you end up with quite a few of them even if you haven't been collecting/playing for very long. Even Rare Candy, which has been supplied to League twice by POP5 and POP8, has only seen 7 releases (including the POP sets). The only cards more commonly reprinted than special Dark and Metal are probably Potion and Switch. (however, Rare Candy has more uses and therefore doesn't tend to fill up binders like special Dark/Metal do. Players can't get enough Rare Candy and new players both love it and need it. When in doubt, you can't go wrong by making Rare Candy a promo again.)
Also, unlike Warp Point, a staple trainer, special Dark and Metal are specialized to only work in a deck using those types (baring rare cases that use them in attack effects). Basic Dark and Metal would have been better because they are much harder to get and they are more easily searched for in-game (being basic instead of special).
If you want a card that always finds its way back into the Modified format, Copycat is a better example. Because it hasn't been reprinted in a while, there's less of them floating around and would have made a better candidate for a league promo. Copycat may not enjoy the usage it did in the past, but it is still a good card and people have taken to it again. It's low reprint frequency combined with it always sneaking back into Modified better serves the rational of special Dark/Metal being good choices because they are always tournament legal.
Not to argue, just explaining why they aren't liked. High reprint frequency vs low usage, and they are specialized in use instead of general.
Looking back at the LPs of last year, how many of them will be ANY good deck-wise going forward?
Snorlax - X
Uxie - O (For one year only)
Arcanine G - X
Dialga G - O (For possibly two years)
Milotic C - X
Claydol - X
Dragonite FB - O (Select decks)
Roseanne's Research - X (Barring reprints)
Felicity's - X (Barring reprints)
Skarmory FB - X
Dusknoir FB - X
Bebe's Search - O (For most likely two years)
Empoleon FB - X
Underground Expedition - O (For most likely two years, but who uses them?)
Butterfree FB - X
Warp Point - O (For one year only)
So we've got 6 useable out of 16. Next year, that will drop to 3 useable, depending on the rotation.
Now look at this season's promos. Almost all will be good for 3 years +. {Speculation} If the next rotation is RR on, and the one after that HGSS on, then the first rotation we'll only lose Azelf. Second rotation, we lose Expert Belt & Spiritomb. {/speculation} Those three are in the first three seasons. The second two season will last a very long time, since they're from our two newest sets. The only one from the old LPs that even compares to the longevity of the new ones is Dialga G. Special Dark & Metal? Theoretically, if TPCi keeps up the rate of re-release, you could play with them forever. How about the other 'tournament staples'? Looking at the big picture, they're flashes in the pan compared to Dark & Steel Energies. Yes, not many competitive players play those types (Sablock & DialgaChomp would like a word)
But the types keep being played. Throughout the game's history, there has been a low percentage (or not so low, at times) that use the types. Added up, that percentage is bigger than those who will use, say, Spiritomb, never mind the other promos like Steelix and Crobat.
Also, I can tell you that new players are JUST as happy to see special Dark & metal as anything else. We've got a new player at league, I offered her a trade for a bunch of rare candies or a bunch of special Dark/Steel. She wanted the Dark/Steel, and we negotiated until I could give her 4 of each. With UD being so Dark/Steel type heavy, there are plenty of new players who either bought the theme deck or bought a few packs and pulled lots of shiny new Pokemon they want to use. Honestly, I don't think I've seen a single pack of UD opened that didn't have a dark or steel type Pokemon in it. Dark & Steel have a mental appeal, since they're supposed to be 'rare.'
