If you need to ask that question, you didn't read the article.
Maybe I read it a little better than you did.
The article carries nothing more than a one line statement that the defense attorney asked for the removal of the ashes because they could "influence" the jury. So we see that counsel believed that it could be detrimental to the defense. I see no evidence in the article to support this assertion.
You however, made the flat statement that the ashes were clearly "prejudicial".
So I repeat my question.
"Prejudicial" to whom and why?
It seems to me that the answer to this question depends on the religious composition of the jury. Do you know the religious affiliation of the jury members?
It's important, because as I mentioned in my first post, a jury carrying significant numbers of non-Catholics of certain persuasions could view the wearing of ashes as ridiculous and could be swayed towards the defense.
In short, the issue is not as clear as you attempt to portray it. I see nothing in the article to suggest that the ashes might be "prejudicial" either way.