When diesel technology took over, it changed the entire Railroad Environment, ending many specialized jobs, entire industries and companies. This is why Steam Locomotives were set aside to be placed on pedestals as monuments to those people and industries. Note that few locomotives were preserved before that time period, as when their usefulness was over, they were recycled.
This is also why early diesels were not preserved; their replacements were/are very much like them, using (basically) the same support systems (manufacturers & technicians). Railroad preservation, as such was nearly non-existent, with many other historical pieces being systematically scrapped (or in the cases of wooden pieces, burned). Actually, in this recessionary environment we are in now, preservation is again becoming less important to the railroads and the public. If the newer (second generation) diesels were radically different, their predecessors would have been preserved in greater numbers.