"On grounds that" is a phrase commonly used to justify a decision or declaration. However, there are several synonyms that can be used in its place to add variety to your writing. These include "due to," "because of," "on account of," "in view of," "owing to," and "as a result of." Each of these phrases has its unique nuance, and choosing the right one can help you convey your point more effectively. For instance, "because of" is a more direct synonym, while "owing to" implies a more presentational or formal tone. Experimenting with different synonyms can enrich your vocabulary and enhance the clarity of your writing.