The only advantage that Foul-Tongue Invocation has is its “reveal a Dragon from your hand” clause. If you happen to reveal a Dragon from your hand when you cast it, you additionally gain 4 life. But outside of Esper Dragons, or any other deck that plays Dragonlord Ojutai, Icefall Regent, or Dragonlord Silumgar, it’s not the most relevant ability.
Overall, To the Slaughter may be better in a planeswalker-heavy metagame than Foul-Tongue Invocation. It also feels like a more playable card. While 4 life is a nice boost in the right context, having the additional ability to do away with a planeswalker probably makes this card better in the long-term.