Segmented armor was never really the standard armor that it is often portrayed to be in movies. Even when this segmented armor was at its prime, it did not replace the long standing use of mail or scale armor which were still very polular.

An important factor about segemented armor is that it is complicated. The lorica segmentata wasn't even very well understood by archeologists who had a hard time literally piecing together the remains they had. It wasn't until later finds and more time passed that they figured it out. This shows that the particular piece of armor was far from being simple to use. It had many pieces which were bound together with lots of leather straps or laces.

This all meant that the armor was more expensive to produce and maintain than its alternatives. This is is a large reason for its eventual decline in the face of more practical and cheaper defensive equipment.