Cowboys from hell had some of the "power metal" sound in it (the album.. not the genre) but it sounded more like Exhorders sound than panteras. When the band released Vulgar... and everything after that it was like a shitty Exhorder knock off.
As to your other question...
promotion... the power of the record labels. Exhorder was on Roadrunner Records. RR is pretty big now, but back in 1988/1989 they didnt have much money at all. Without money you cant promote the band well, you cant send them on any huge tours (although exhorder did to two world tours)... in short you cant do anything except hold them back.
Pantera on the other hand was signed to Atco, which was owned by Elektra... AKA Atlantic Records... a MUCH bigger label with a MUCH bigger budget.
do the math