Sadly it seems like it is many times easier for politicians to push a soda tax (which also serves as a political gesture on its own that can be framed as being

Sadly it seems like it is many times easier for politicians to push a soda tax (which also serves as a political gesture on its own that can be framed as being for the good of the every day people and against the controlling influence of Big Soda) rather than undo decades of agricultural policy, which would also be controversial amongst more rural bases. As you point out, it completely defies elementary logic - but politics and policy seem to have preoccupations and incentives that are orthogonal to elementary logic.