Influential as they may be, Celtic Frost isn't gonna draw for shit. Who's their audience? Older guys who were into 'em back in the day and people seriously into the metal scene who are willing to dig deep enough to discover them. While I'm sure the audience they'd draw would be seriously enthusiastic about seeing them, if they were to tour the US now, in 2006, they'd be doing small to medium sized clubs, playing to a few hundred people. And that's with good openers. They weren't a huge commercial success then, and they're certainly not now.
Opeth is on a pretty big label and has enjoyed a fair bit of commercial success for a metal band in the past few years. They headline good sized venues and draw a decent crowd. Regardless of who occupies the more important or influential place in metal history, Opeth is gonna be the bigger draw. Celtic Frost will draw serious metal scene types looking to see influential underground legends and older guys seeing a nostalgia act.