I feel like this change (replacing shiism by shia) should have been implemented three years ago when it was first suggested. I just thought it was a mistake when I joined the site, e.g. shiism is inconsistent with sunni, and "Shia" is used in shia-sunni.
1. It's what people will search for, by a huge margin
Here's the Google Trends (e.g. here) results, showing the popularity of search terms. It's what people will search for as a keyword in Google, and it's what's best for search engine optimization. Thus, by keeping shiism instead of shia, fewer people will find Islam.SE.
What Kaveh said then is still true---it's the word (and hence search term) that people favor to a huge degree.
Here's a more inclusive Google N-gram results when we include "Shia" (with/without the apostrophe) and a bunch of variants. "Shia" is the clear favorite here.
So people will find more search results by using the keyword
shia instead of
2. Tags are about indexing, not grammar
Tags are for keywords. They're not intended to be grammatically perfect. Moreover, the StackExchange software prevents grammatically correct tags by e.g. forbidding characters such as
' and capital letters (e.g. allah).
3. Wikipedia favors "Shia Islam"; we don't need the "Islam" part
As Bleeding Fingers points out, Wikipedia Shiism redirects to Shia Islam, making "Shia Islam" the master. We don't need the word "Islam", as this is a site about Islam (what else is it going to mean?). The first line of that Wikipedia page says:
Shia ... is a branch of Islam ...
On this page the word "Shia" appears 154 times, whereas the word "Shiism" appears 9 times.
Shia Islam is furthermore the title of a whole Wikipedia category.
The etymology section on Wikipedia seems to have changed too:
The word Shia ... is the short form of the historic phrase shīʻatu ʻAlī (شيعة علي /ˈʃiːʕatu ˈʕaliː/), meaning "followers of Ali" ...