He was redistricted. He wouldn't win in his "new" district --- not enough concentration of brain dead genetic Rats.
Representatives who are expelled from Congress are PROHIBITED from running for that office again. They were interviewing the last guy who was expelled (Rep. Ozzie Myers in '80) and he said he does a lot of behind-the-scenes work on political campaigns but he is barred from ever seeking elected office himself. (The corrupt 'RAT says he's "not bitter" about this fact)
On the other hand, if Traficant is simply imprisoned because of his conviction then he could techinically continue to serve in congress. As long as he's re-elected then there is nothing to prevent a convicted felon from currently holding office. I think a mayor of Boston was re-elected from prison back in the 40s or something.
Impeachment is slightly different than expulsion-- you are prohibited from holding office in the future only if the articles of impeachment specifically say so. For instance, Alcee Hastings was impeached and then removed as a federal judge...but the impeachment was because he held appointed federal office and they wanted to ensure he would not be appointed as judge again. Later he ran for congress and won thanks to stupid Democrats.