Differences between the introduction of a sandwich thesis and a paper
From a structural point of view, there is no major difference. An introduction should set the work in a wider context starting by establishing the wider context and showing the need for the work done and identifying the knowledge gap that is filled. What is different concerns the matter since a thesis introduction should focus on the thesis not the individual studies (articles/manuscripts).

If the articles/manuscripts all are very tightly tied, then it is probably easier to write the introduction. If the articles are more disparate, which is not uncommon, then trying to tie everything together can be more challenging.The stumbling block for most is that they have not thought about the wider perspective and some do not even manage well to include it in the articles/manuscripts themselves. So I would say that the difficulty does not lie with the fact that it is a thesis introduction per se but rather that many do not see the forest for all the trees while they write the articles/manuscripts to be included in the thesis.