Fear of Change
Moving from ECC 6.0 to S/4HANA is a Migration not an Upgrade and the Big opportunity for simplification and Fit To Standard, the best practice approach to package implementation.
The Clean Core and Side by Side enhancement keeps the lower cost promise of package implementation and the Platform as a Service for justified enhancements. No longer mixed together like ECC.
The move from on premise hardware to an outsourced data centre is small change since virtualisation enables lift and shift when you need to do that, every 4-6 years. Obviously outsourcing will be a new supplier relationship so that's where RISE simplifies that change to SAP as the single point of contact.
Doing both at once enables the reuse of SAP reference architecture and proven Application Lifecycle Management tools, so lowers risk and cost.
Multiple steps are more cost, more change, and more risk. The hard part is always the Business Transformation and people becoming comfortable with the rationale and process for change. Get good help and do it right first time for the right reasons..