Here are a few thoughts on why I think Protestants don’t like theosis as a soteriological category.  These are a few claims that I’ve heard about why it’s not good. Some are more difficult to overcome than others.

1) It is unfamiliar.

2) It unseats justification: As a competing soteriology, it impinges the sole place to be held by justification, especially because justification language isn’t really used at all to describe theosis.

3) It is synergism:  Not only does it replace justification, but it smacks of a works-based salvation. 

4) It is idolatry: It confuses the created with Creator.  That is, it repeats the sin of Adam and Eve.

5) It is mysticism: Closely related to #4, is the idea of fusion of the created with the creator (a la neo-platonism).  Whereas #4 is ideological, #5 relates to ontology. 

6) It sounds like Mormonism.