Bear in mind this was a period when somebody building a bike was investing more in the forks than in the frame, also often more in the disc brake system than in the frame. So the frame became less important, and a brand like Kona was looking to make savings on the frame.
So if you look at the Explosif, in 1998 the front triangle was Reynolds 853, and the stays were Reynolds 735, the mix recommended by Reynolds. By 1999, the '853' Explosif had gone down to generic 4130 non heat-treated stays. After a couple of years of failing to sell scandium Explosifs at very high prices, they reverted to steel and from 2002 to 2004 they built what is known as the True Temper OX Platinum Explosif. However only the top and down tubes are True Temper OX Platinum and every other tube is generic 4130. They did have butted stays in 2002/3, but by 2004 they went to plain-gauge stays.
I can't really answer your question because I have never ridden any Explosif later than 98, but it's interesting that the disc system and perhaps the need to compete with the dominant aluminium changed the way steel frames in general were designed. Not just lowering cost, but gradually increasing weight, and also transferring weight backwards - the front triangle staying very light, but the stays getting heavier. But I would imagine the move to plain-gauge stays in 2004 was merited rather than for economy - they needed to be stiff to withstand braking forces and to give a ride as solid as aluminium. So maybe they found the combination of disc brakes (used for the first time in 2003) with butted stays (used up to 2003) had too much flex for optimum performance.
I don't know that, it's just speculation - manufacturers never come clean over this sort of thing (after 2004, Kona stopped giving details of frame specs - to be fair, they had been exceptional in being so open up to then). Rocky Mountain went the other way when they moved the Blizzard from 853 to Columbus Zona - they made it lighter. Not that Zona is lighter than 853, but they chose a lighter spec. I think latter day Explosifs weigh c5lbs, as do VooDoo Bizangos, and people speak well of them - they're built that way for modern purposes. I guess the so-called OX Platinum Explosifs were a step on the way to that.
I always enjoy reading your posts Anthony
On a side I always thought that the rear stays on my 1999 Caldera were double butted - not that it matters either way.
On the look out for a 20'' 1998 Kona Explosif
No mention of 725 stays though