Table:
|  |  | Year Ended December 31, |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
| Subscription revenue | $542,968 | $473,052 | $396,764 |
| Percentage of subscription revenue to total revenue | 94.2% | 87.9% | 82.3% |
| Professional services revenue | $33,555 | $64,839 | $85,221 |
| Percentage of professional services to total revenue | 5.8% | 12.1% | 17.7% |
| Total revenue | $576,523 | 537,891 | 481,985 |
Paragraphs:
The following table sets forth our sources of revenue for each of the periods indicated (in thousands, except for percentages):
Subscription revenue increased by $69.9 million, or 15%, in 2019 when compared to 2018. Subscription revenue growth on a constant currency basis increased 16% in 2019 when compared to 2018. The increase was attributable to new business, which includes new customers, upsells, cross-sells, and renewals from existing customers.
Professional services revenue decreased by $31.3 million, or 48%, in 2019 when compared to 2018. The decrease of professional services revenue is attributable to the continued migration of implementation services to our global partners.
Subscription revenue increased by $76.3 million, or 19%, in 2018 when compared to 2017. The increase was attributable to new business, which included new customers, upsells, and renewals from existing customers. Professional services revenue decreased by $20.4 million, or 24%, in 2018 when compared to 2017. The decrease of professional services revenue is attributable to the execution of our strategic initiative to migrate much of our implementation services to our global partners.
Question:
Why did subscription revenue increase by $76.3 million between 2018 and 2017?