Jut not a reason you can... you know... say? Got it.... great reason. How could anyone argue with such a slam dunk case.
I had thought the many follies of 3.x design well know. The fact that they changed it from 3.0 to 3.5 is a little bit of hint there. IIRC, initially they had flat out immunity to anything that was of the appropriate type (ex fire) or not of the requisite plus value (+3 weapon or greater to damage at all). This made monsters more of a "you must by
this high to take this ride" type of challenge and led to the 'golf bag syndrome' of PC's touting around a 'golf bag' full of different types of enchanted weapons for the appropriate type of encounter. 'A par 4 with a sand trap? Better pull out the 9 iron with sand wedge as backup'. 'Demons you say? better pull out the cold iron plus 4 battle axe, keep the silvered, blessed sword of good on hand in case we're walking into the blood war and some devils show up .'
3.5, iirc, went with a more measured 10/fire, where the first 10 fire damage was ignored, but any damage beyond that was taken by the creature. The same for the required plus of the weapon. The problem here was not only slightly more math involved in calculating from every hit, but the static resistance tended to be either trivial beyond a certain point, or overwhelming before another point. Only in between the two points was it really effective, and still led to the 'golf bag syndrome' and 'magic item Christmas tree' effect that was complained about in that edition.
Applying such to 5e, with the design on much less reliance on magic items than previous editions and much of the damage increase for PCs coming from multiple discrete attacks, would not be a desirable solution, imho.