Mike H. makes an interesting point. Potential passengers may have been elsewhere and boarding ships from other European ports not affected by coal strikes.
However, at this time passengers may well have been avoiding "The Olympics". During her maiden voyage the
Olympic almost crushed a tug. Three months later she was significantly wounded and required an extensive renovation that resulted in her missing three round trips and in February of 1912 she lost a propeller blade. This may all have helped to wilt her popularity of her first few months in service. I haven't got specific statistics, but in some late 1911 or early 1912 trips Olympic carried a total of 400 passengers throughout all her classes.
On April 3, 1912, a week prior to Titanic's departure, Olympic sailed from Southampton. She arrived in New York on April 10, with the exact same complement of 1st class passengers, that Titanic would have on April 14, 1912 -- 324 (including Nourney, who transferred from 2nd).
It wasn't just Titanic that was so under filled, it was Olympic as well. Perhaps it wasn't just those ships, perhaps all other ships still in service were also experiencing a lack of passengers.
Daniel.