Spiderman said:
Less common, afterthought, it all comes down to not considering Hymn as a viable choice for a dual color deck.
When I make a discard deck with two colors, I always pull out the Hymn and put them in my "cards to use" pile when getting the cards together, period.
Any "discard deck" I've seen has had black as the more common color. This is simply because Hymn, Duress, Mind Twist, Hypnotic Specter, and almost all the other decent discard spells are black. Control decks that are not entirely discard-based and use Hymn tend to be monoblack (like Necro and other MBC designs). Hymn doesn't fit very well into multicolored decks. I don't know whether that means it should be considered a viable choice or not. But, like Fork, it tends to stay within its own color (and is sometimes used in two-color decks, but I'd say there's a noticeable difference).
The only reason why I'm bringing deck tuning into this is because you seem to think it's difficult getting dual mana out for when you want to use the card, in general I'm simply saying it's not hard enough to warrant excluding a dual colored mana spell in a dual deck.
Difficult is a relative term. It is more difficult than only one of the mana needing to be of a specific color. It isn't an insurmountable obstacle, but no one is saying it is.
And all I'm saying is that the drawback is negligible.
It isn't negligible. If Hymn costed 1B, it would be easily splashable in three-color decks. If Counterspell costed 1U, it would be Mana Leak without the drawback. People use Mana Leak, and I've seen it used over Counterspell on occasion (of course, they're usually used together). If the mana difference were truly negligible, Mana Leak would be considered chaff compared to Counterspell. If Morphling costed 4U instead of 3UU, then it would be available to various multicolored decks (fortunately, three of its abilities also require blue mana, holding it back some more). The mana cost of a spell does matter.
TomB: I don't think it's the land limitations back then (although I think having the dual lands or pain lands would have still helped than hurt) My perception may be more shaped by Fork was restricted for most of these years and the chance of getting it anyway was pretty slim overall, so when I did get it I could cast it.
I don't think it's land limitations either. The biggest advancement there has probably been the Onslaught fetchlands, which can grab the right dual lands at the right times.
Fork wasn't seeing much use while restricted, and that doesn't seem to have changed with unrestriction. I also haven't seen it in any Legacy decks. But if the effect is still desirable, we'll probably be seeing Twincast used more than Fork (since it's blue).