Table:
|  | Years Ended December 31, |  |
| --- | --- | --- |
|  | 2019 | 2018 |
| Revenues |  |  |
| ACI On Premise | $579,334 | $576,755 |
| ACI On Demand | 678,960 | 433,025 |
| Total revenue | $ 1,258,294 | $ 1,009,780 |
| Segment Adjusted EBITDA |  |  |
| ACI On Premise | $321,305 | $323,902 |
| ACI On Demand | 66,501 | 12,015 |
| Depreciation and amortization | (122,569 ) | (97,350 ) |
| Stock-based compensation expense | (36,763 ) | (20,360 ) |
| Corporate and unallocated expenses | (104,718 ) | (92,296 ) |
| Interest, net | (52,066 ) | (30,388 ) |
| Other, net | 520 | (3,724 ) |
| Income before income taxes | $ 72,210 | $ 91,799 |
| Depreciation and amortization |  |  |
| ACI On Premise | $ 11,992 | $ 11,634 |
| ACI On Demand | 34,395 | 31,541 |
| Corporate | 76,182 | 54,175 |
| Total depreciation and amortization | $ 122,569 | $ 97,350 |
| Stock-based compensation expense |  |  |
| ACI On Premise | $ 7,651 | $ 4,348 |
| ACI On Demand | 7,995 | 4,338 |
| Corporate and other | 21,117 | 11,674 |
| Total stock-based compensation expense | $ 36,763 | $ 20,360 |
Paragraphs:
The following is selected financial data for our reportable segments (in thousands):
ACI On Premise Segment Adjusted EBITDA decreased $2.6 million for the year ended December 31, 2019, compared to the same period in 2018, primarily due to a $5.2 million increase in cash operating expense, partially offset by a $2.6 million increase in revenue.
ACI On Demand Segment Adjusted EBITDA increased $54.5 million for the year ended December 31, 2019, compared to the same period in 2018, of which $46.4 million was due to the acquisition of Speedpay. Excluding the impact of the acquisition of Speedpay, ACI On Demand Segment Adjusted EBITDA increased $8.1 million, primarily due to a $18.3 million increase in revenue, partially offset by a $10.2 million increase in cash operating expense.
Question:
Why did ACI On Premise Segment Adjusted EBITDA decrease between 2018 and 2019?
Answer: due to a $5.2 million increase in cash operating expense, partially offset by a $2.6 million increase in revenue.