It's not - the Republican party didn't grow its base. In fact I don't even think Trump knows why he succeeded. He is a bigot and a moron and I cannot see him succeeding, because like most demagogues his answer is to point the finger of blame at the vulnerable.
But like Pauline Hanson and Nigel Farage he taps into a valid and serious vein of discontent that can no longer be ignored. Brexit did the same: people don't want to throw their country under a bus, but they're crying out for fairness. People didn't vote for Trump as much as they voted against the status quo - because that status quo has been fucking people over since the GFC. Wealth inequality is the highest it has been in a century, since 1929. The year the stock market crashed and shored in the Great Depression (and, eventually, war).
All over the world asset bubbles (be they in housing, education, healthcare, stocks) have been allowed to fester and swell, and governments have enacted policy after policy to gut the middle class in favour of preserving the wealth of these few. But it's all been a fallacy. Real incomes of everyone but the rich have stagnated for a decade. "Growth" has been a lie, because despite "growing" GDPs the only people getting rich are those who happened to be on the right side of these bubbles central banks so blithely created. People have been sold the bullshit of trickle-down economics and now they've realised they were had. Wealth inequality is a ticking time bomb. The world should never, ever have allowed this to happen.
We need to burst our property bubble - stop feeding it with bullshit inequitable and greed-based wealthfare handouts to the rich. We need to stop favouring our retiring generation with policy that prevents the financial stability of the young, or we are in serious trouble. We need to have children, and support families, rather than relying on the band-aid solution of over-immigration (don't get me wrong, immigration is a great thing, but we are way overdoing it right now and this places an extreme strain on infrastructure and contributes to youth unemployment).
I hope this does lead to change. Trump is frightening but I hope he brings a meaningful shift.