I’ve worked on a few migration / rewrite /port projects in my time. They almost universally had no technical merit in and of themselves. From a developer’s perspective, a rewrite of an existing system basically means taking a stable, tuned application, freezing all new features, and writing it again in a language that’s newer, possibly more buggy, and probably less familiar to many of the team members, and delivering at best exactly the same features as the existing system, while taking several months to do it, with a risk of introducing new bugs along with it.
Migration projects are almost always done for political or cost reasons. Either the company has decided on a strategic plan to ‘go Java’ or ‘go .NET’, or (possibly less high profile, but potentially more significantly), is having a progressively harder time retaining developers with the skill set needed to support the existing system. Technology can also be made a scapegoat – so if a system is delivered that runs too slowly, or crashes a lot, the entire platform it happened to be developed with is proclaimed as ‘slow and unstable’, and the baby is thrown out with the bathwater.