Perhaps it was a lack of resources and means to create the poly-laminate composites the suits we're made from. They did only bring what they could carry from the military base and what was in the bunker before they expanded it. Years of scavenging, trading and wiping out raiders. Going out to push back and destroy as much of the Master's Army as possible. Them playing a smaller role in the wastes before finding out about the Enclave. Helping the NCR take Navarro before them getting into an all out war. All that has probably strained what parts they had on hand and forced them to use different resources for new suits.