Untrue.
Divine Intervention was released by Slayer in 1995 and became their highest charting album ever. Everyone forgets about DI because it followed SitA, which was a classic, yet DI was very successful.
In 1998 Slayer released Diablous in Musica, which was a disappointment, yet it still charted high. It is probably the closest Slayer ever came to changing their sound. It was still successful, though not as successful as DI.
In 2001 Slayer released God Hates Us All, which despite having the misfortune to release on the same day as the 9/11 attacks, charted high. It was generally considered a return to form and sold more albums than DiM. It was successful as well.
Megadeth released Countdown to Extinction in 1992, which was their most successful album. Megadeth released Youthanasia in 1994, which was their fastest album to platinum status ever. In 1996 Megadeth released Cryptic Writings, which got to number 10 on the charts and had a single which charted higher than any Megadeth before it.
After that, Megadeth released Risk in 1998 which was a commercial and critical failure. This was Megadeth's most drastic change of sound and they paid the price for it.
Bottom line, your perception that "No metal band was selling records in the mid 90's except Metallica and Pantera." is patently false. Slayer, Megadeth and Metallica were all selling well. The only one of the big four that was not was Anthrax....
Care to guess why?