He was pretty much the only finalist candidate that proposed cutting taxes, and not just by a bit, Trump by his nature wanted the greatest change
Jeb Bush: https://taxfoundation.org/details-and-analysis-governor-jeb-...
Marco Rubio: https://taxfoundation.org/presidential-hopeful-marco-rubio-a...
You can compare all candidate plans here: https://taxfoundation.org/comparing-2016-presidential-tax-re...
In fact it was Ben Carson who proposed cutting the tax rates the most (corporate tax rate -> 14.9%).