Conversation so far:
who is elected for a six year term
Senators serve six year terms.
what is the age qualification to run for Senator
To be a Senator, a person must be aged 30 or over
does the candidate for senator have to be a us citizen
Candidates for U.S. Senator must have been U.S. citizenship for at least nine years before running.
how many senators have been expelled in the senate's history
Fifteen senators have been expelled in the Senate's history

Question: why
Self-contained rewrite: why were us senators expelled in the past