May 2 (Bloomberg) — Even the strongest job growth in more than two years isn't enough to entice more people into the labor force, one of the biggest conundrums of the U.S. economic expansion.

Labor force participation, measuring the share of the working-age population either employed or seeking a job, declined in April for the first time this year, helping drive the unemployment rate down to 6.3 percent, the lowest level since September 2008. At 62.8 percent, the participation rate matches the lowest level since March 1978 and is down from 63.2 percent in the prior month.

The plunge in participation came as fewer people entered the work force, while the number of people who have given up the search for employment remained close to the average for the last year. These so-called discouraged workers totaled 783,000 in April compared with 835,000 a year earlier.