What's the biggest blocker for legacy large companies IT to switch to cloud technologies?

Here is a recent question in an online forum. My response follows.

A few years ago, a new CIO took over our organization, a multinational supply-chain company and had a clear cloud-first mandate. The team was able to get the stakeholders aligned on a cloud (SaaS, PaaS, IaaS) based portfolio strategy.
I led an extensive assessment of the application portfolio, reviewing the different dimensions of individual platforms and capabilities.

1. Some applications were clearly SaaS candidates since the vendor offerings had matured
2. A few were good candidates for PaaS, especially where the organization had the capability to manage the service
3. Due to network, latency, data volume, legal and other constraints, some applications would have to remain in IaaS (Private, Virtual Private Cloud)
4. Some legacy applications had clearly not been designed to be virtualized and would not benefit from a move to the cloud (even to IaaS). These were running on old/proprietary hardware infrastructure and would have to be redesigned (and not just ‘migrated’)

If we set aside politics, turf wars, legal and privacy constraints, one will still encounter genuine technical constraints (#4 above) that prevent an application move to the cloud.

One might argue that even those applications can be ‘migrated’ to a cloud with some refactoring. However, the cost of such migration may not justify the benefit. Doing so merely to satisfy CxOs ego to say ‘we are entirely on the cloud’ is bound to be counterproductive.