They always were dime. The founding fathers set it up as an oligarchy. The pieces they left out were filled in by the majority of presidents 1800-1900. Then you step into Wilson's presidency and shiiiiiiit gets weird. So it's not the last 25 years, it's been like this from the start.
To "fix" it, you need the Republicans and Democrates to both go the way of the Whig, Democratic Republicans, National Union, and Federalist parties... I opted to only list the parties which were successful enough to have a president elected from them. We have a two party system simply because that's who's elected. It's not mandated.