The primary drivers of the increase in our revenues are revenues from new contract awards, growth on existing contracts and the acquisitions we completed in the prior year. These increases were offset by contracts and tasks that ended during the year and reduced scope of work on some contracts including contracts with variable material purchase requirements. We expect revenues to increase in 2022 due to our recent acquisitions, growth on existing programs and new contracts.
The increase in cost of services was primarily due to increases in revenues. As a percentage of revenues, direct labor costs were 50% and 49% for the years ended December 31, 2021 and 2020, respectively. As a percentage of revenues, other direct costs, which include subcontractors and third party equipment and materials used in the performance of our contracts, were 36% for both the years ended December 31, 2021 and 2020. With COVID-19 mitigation protocols being reduced or lifted, direct labor has been impacted as employees have begun utilizing paid time off at a normalized level. Profitability has increased due to higher program profits including improved award fees as compared to the prior period.
The decrease in general and administrative expenses was primarily the result of changes in the classification of certain indirect cost allocations of approximately $30.1 million. These decreases were partially offset by a return to more normalized indirect spending compared to 2020, which experienced reduced travel, conference expenses and other indirect expenses due to the impacts of COVID-19. As a percentage of revenues, general and administrative expenses decreased for the year ended December 31, 2021 as compared to the same period in 2020. We expect general and administrative expenses as a percentage of revenue to increase slightly in 2022, due to higher amortization expenses and continued return to normalized indirect spending.
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