Because (Thanks to this) of the new Republican platform (programme), Reagan was able to attract democratic voters.
Democratic voters based on "New Deal coalition" (1930s to the end of 1970s) : poor whites trade unions minories (jews, catholics, racial minorities) intellectuals
Those people who traditionnal voted for the democratic party. The poor whites, the 1960s, the democratic in racial minorities, so poor whites started to feel that the main focus was minorities, so they felt neglected. In fact, these people moved to reagan party, was the candidate of white traditionnal values. These people actually voted to Reagan. That's why those people are called the "Reagan Democrats".

In political strategy, the main group is "reagan democrats" (he is very very important) because others will vote to their candidate party so only "reagan democrats" can change their vote.

Third reason, demographic reason : ageing population.

When you have a population gets older, the population usually vote for the conservative party. Ageing population : conservative US Internal migration : East and West coasts vote traditionnally for the south & west conservative

SECOND PART : The domestic situation in the 80s

"The REAGANOMICS"
It is R.Reagan's principles of the R.Reagan policy.
The Principles : the New Deal consensus (1932 – 19680) based on 2 ideas, inspired by J.M KEYNES (Keynesion principles): one the one hand, tax & spend "economic policy" (high taxes and high spending) So the government was the engine of the economy. (build school, welfare state..ect..) the result is that there is a "Big government" "demand-side economics" the government is here to help companies but helping the supply so the consumers. In particulary by the "welfare state" (unemployment benefits) to boost consumption. Reaganomics where inspired by Milton FRIEDMAN, was indeed a return in the "ruthless capitalism". The first principle of the