We do not use the triangle for more than one reason. The foremost reason is the triangle represents the Trinity (Father, Son and Holy Ghost).
We do believe in the Trinity but we do not believe in a metal plate that is said to represent the trinity. In the 10 Commandments it says: Thou shall not make unto thee any graven image, or any likeness of anything that is in heaven above, etc. . .
No material is valuable enough to even begin to represent the holiness of the supreme one.
The Commandments say: For I the Lord thy God am a jealous God, etc. . .