The issue arises from Precision's strength, and how most other masteries were nerfed. Precision is so strong right now on anyone with either damage type (let alone anyone with hybrid damage) that it is just the go to mastery for damage amplification overall over the %pen masteries. Add in the fact that Thunderlords then scales on it indirectly, and it's fairly obvious what everyone would want.
And 20 seconds between trades? Hell, naturally you don't want to trade until all of your abilities are up, so this limits trades to at least every 10 seconds on most champs. And obviously you are not trading on CD, you go oom way too fast unless you are dominating. This means that waiting for a 20 second mark for Thunderlords, which rewards the quick "burst them and fall back before they can retaliate" trading pattern 90% of champs prefer for trading, is even better rewarded. How often are you going to keep a Deathfire Charge on someone for 6+ seconds? This is what it takes for it to be stronger than Thunderlords, just assuming level 1 trading. TL's base damage scaling (which no other mastery has a base damage scaling per level, only scaling damage if only in the form of max hp or ratios), means it naturally has an advantage. Honestly, unless they change Thunderlords to not proc on Auto attacks (or for an auto attack to only apply 1 stack, and abilities two, and then TL be changed to apply at 6 stacks), it will be pretty hard for Thunderlords to ever be fully removed from the choice, just because for some champs/abilities, it just wins hard. Example, MF can proc it with just her E, and loves Precision for the early game penetration for her E's good base damages. She then proceeds to build heavy AD, giving Thunderlord the 30% bonus AD scaling to run off of. Guess what? She wins out on damage hard with a solid E poke doing so much.
Combo this with how some champs have a natural "I can apply the entirity of Thunderlords with literally one ability/auto attack!" thing going for them, Thunderlords just wins compared to Deathfire Touch.
If Deathfire was changed to have it's AoE scaling be only if the AoE hits multiple targets, it probably would be more worth it, but as is most AoEs only hit a single target anyway if not an ult, just because of size. Meanwhile, change Thunderlords to have less damage, but the AoE size to actually have a pretty large AoE. Now it's intended purpose of being the AoE wombo combo mastery is in, and it's less of a "Oh, I just get an automatic 10-180 + 30% Bonus AD and 10% AP on all fights upfront" mastery. Perhaps make it so that the mastery can be applied once per champion every 30 seconds, with faintly lower damage, rather than having a global 20 second CD before you can apply it to any champion. Combo that with it having a larger AoE size, it would solidify it as the AoE wombo mastery, and remove it from the "General all purpose damage increase" mastery. Lets pretend we halve its damage, but AoE is nearly a lane in width. Very large, low damage proc. If you proc it on 2 champions, it matches it's old damage to a single champion, but it has had the potential to apply it overall twice. If you AoE wombo it, you can get up to 2.5x damage of the old Thunderlords, but it requires proccing 3 hits on ALL 5 enemy champions. Combo with a change so each cast of an ability can only proc one hit for the purposes of Thunderlords once per champion to prevent people with multi hit AoEs just instantly applying it to an entire team, and it's a much better balanced mastery, with clear strengths and weaknesses.