It's not they don't run on anybody else's image. They might. It is that they are difficult to get to run on other images.

Have you tried porting Smalltalk apps between different vendors' Smalltalks? Problem is there is no standard Smalltalk base-image. That is caused by the fact it is so easy to modify everything in the image you have and then save it and give it to others. Others then start developing against that.

You can try meticulously to avoid this problem but it takes special attention.

Contrast this with Java. You can run any Java program that was developed on a given version of JDK by using that same or later versions of JDK.