If no PRIMARY candidates are running for the same office in one party, there is nothing to select.  The Primary candidate will win as (S)he is unopposed.  Suppose all Republican Candidates were rnning in primaries unopposed.  You could only vote Democrat because the other party had already picked their candidates, i.e, no-one chose to run against them. It was not the party's choice, it was the lack of candidates who wished to run against the incumbent.