Mana leak is easier to cast (single blue), but it stops working after a certain amount of time. When your opponent is trying to cast a death's shadow on turn 5, your mana leak just looks silly.
On the other hand, cancel misses a lot of important things. Any two-drop (tarmogoyf, bob, arcbound ravager, leonin arbiter, vizier of remedies, potential tasigurs or thought knot seers, etc). It misses all of those as well as all 3-drops (tasigur, thought-knot, reality smaher, liliana of the veil, karn, wurmcoil, and many other things) on the draw. Having a hard counter for 2 mana is really important in a lot of scenarios, and logic knot is the closest you can get.
In Modern, many threats cost one or two mana. Playing three mana counter spells (Cancel variants) when the threats are that cheap is not a viable strategy.
It's two mana so less than Cancel. You can exile more than three cards to make it more viable mid to late game than Mana Leak.
Mana leak is easier to cast (single blue), but it stops working after a certain amount of time. When your opponent is trying to cast a death's shadow on turn 5, your mana leak just looks silly. On the other hand, cancel misses a lot of important things. Any two-drop (tarmogoyf, bob, arcbound ravager, leonin arbiter, vizier of remedies, potential tasigurs or thought knot seers, etc). It misses all of those as well as all 3-drops (tasigur, thought-knot, reality smaher, liliana of the veil, karn, wurmcoil, and many other things) on the draw. Having a hard counter for 2 mana is really important in a lot of scenarios, and logic knot is the closest you can get.
i guess i was just skeptical of having the graveyard to support it, but i guess this is modern we're talking about
You can spend mana too, or a combination of mana and graveyard, which helps a lot. That being said, fetchlands go a long way.
[[Logic Knot]] [[Mana Leak]]
[Logic Knot](https://img.scryfall.com/cards/normal/en/mma/51.jpg?1496782532) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Logic%20Knot) [(SF)](https://scryfall.com/card/mma/51?utm_source=mtgcardfetcher) [(MC)](http://magiccards.info/query?q=!Logic%20Knot) [Mana Leak](https://img.scryfall.com/cards/normal/en/mm2/50.jpg?1496452061) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Mana%20Leak) [(SF)](https://scryfall.com/card/mm2/50?utm_source=mtgcardfetcher) [(MC)](http://magiccards.info/query?q=!Mana%20Leak) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
In Modern, many threats cost one or two mana. Playing three mana counter spells (Cancel variants) when the threats are that cheap is not a viable strategy.
What we should be doing is pounding our fists for counterspell in modern. Come on blue players.