To make a tactical vote, you need an estimate how much votes each candidate is probably going to get. Only then, looking at the numbers, you can decide that it will be tactical to vote Y instead of X -- because X has almost no chance to be elected, and although you would prefer X to Y, you also prefer Y to Z.

Now, how do you get these estimates? Most people get them from the media.

And this is the problem: if you own the media, you can manipulate the election estimates, which lets you indirectly manipulate the tactical votes of citizens. Such a manipulation can easily become a self-fulfilling prophecy. If you make everyone believe that "X has no chance of winning, because only 1% of people would vote for X", many people will vote strategically for Y... and at the end you may be actually proven right, because only 1% of people will vote for X.

Shortly: Tactical voting gives too much power to media owners, because they influence the information people use for voting tactically.

To make a tactical vote, you need an estimate how much votes each candidate is probably going to get. Only then, looking at the numbers, you can decide that it will be tactical to vote Y instead of X -- because X has almost no chance to be elected, and although you would prefer X to Y, you also prefer Y to Z.

Now, how do you get these estimates? Most people get them from the media.

And this is the problem: if you own the media, you can manipulate the election estimates, which lets you indirectly manipulate the tactical votes of citizens. Such a manipulation can easily become a self-fulfilling prophecy. If you make everyone believe that "X has no chance of winning, because only 1% of people would vote for X", many people will vote strategically for Y... and at the end you may be actually proven right, because only 1% of people will vote for X.

In short: Tactical voting gives too much power to media owners, because they influence the information people use for voting tactically.