Why did demographers stop placing empasis on the decline in mortality as the cause of low birth rates? what other alternative model was suggested?

They stopped placing emphasis on the decline of mortality because it fails to explain all the aspects of the problem. Sure, it is one of the reasons, but it can't provide the full picture. There are two alternative models: cultural influence, and economic and investment. Both of those models include education, as it strongly influences cultural models and improves chances of economic progress.

It is important to note that all of these models (decline in mortality, cultural influence, economic and investment) are partial and complementary: they should function together, as each of them explains only one aspect of the phenomenon.