It was to balance it with Cursed Flames, but both Ichor was nerfed and Cursed Flames was buffed, and ichor deserved the nerf anyway.
Look at 1.3 Cursed Flames. It literally dealt 6 damage per second to enemy targets. Ichor was a flat damage increase of 10. To compare that, in order for Cursed Flames to make a difference on your DPS larger than what Ichor can offer, you’d have to attack quite a bit less than once per second, which would literally never happen in any scenario. On top of that, literally even Frostburn had a stronger DoT debuff than cursed flames.
1.4.1 Cursed Flames is 24 damage per second, and Ichor is a flat base damage of 7-8. If you’re attacking three times per second or more, Ichor is better. Which is still the majority of viable weapons - and almost any build if you use minions on the side.
And there’d be some problems with just buffing Cursed Flames even further. One, Cursed Flames needs to not outclass Venom, a debuff which comes later. And two, you shouldn’t be able to completely finish off an enemy after hitting them once. That’s the issue with DoT debuffs - you have to draw a line on them
And Ichor *was* notoriously overpowered, to the point that you would (and this still is the case) use it into the endgame, and definitely deserved a nerf. It was the single largest selling point of Crimson over even Vampire Knives I can’t think of a single other debuff that’s being used for such a long period of time, let alone with such a massive effect on your DPS on Ichor.
If you don’t think the effect was that big, look at Megashark - it hits about 8.5 times per second for roughly 32 base damage with Musket Balls - about 272 DPS. Add *another 10* from ichor, applied by a melee weapon with an ichor flask, a bow with ichor arrows or a golden shower, suddenly your DPS is 357. That’s a bigger percentage damage increase than any accessory, or armor piece, or DoT debuff not caused by pillar weapons. This same situation applies with anything that attacks reasonably fast - so basically the majority of good weapons.
Anyway, now that Ichor and Cursed Flames are closer, they both have a valid role. Ichor is still a huge boost to single target DPS while Cursed Flames can deal somewhat smaller damage but can effect targets you aren’t actively hitting, including crowds.
TLDR it was perfectly reasonable to nerf ichor.