The "Keltics" pronunciation is what would be correct when speaking in a Celtic language. So, many English speakers have switched to that.
But sports teams like the Boston Celtics and Celtic F.C. of Glasgow have been known as "Seltics" for so long, it would be too strange to change their pronunciation.
I recall in the early or mid 1990s, NPR's "All Things Considered" did an April Fools Day story about the Boston Celtics signing a star prospect from Ireland who signed on the condition that the team change the pronunciation of its name to "Keltics."