Leaving aside the obvious "the people who would put rules into place to stop this are the people who benefit from not having a rule in place aspect", there actually is a good reason. Life is complicated, and looking after a country moreso. The circumstances someone makes those promises in are rarely the circumstances in they have to act on them.

A simple example; you pledge to reduce taxes. And you mean it! You've run the numbers, and you've figured out how to do it. The people get behind you, you get voted in, and, you start figuring it out. Then a bomb is dropped, literally. Now, you need money to rebuild, but you worked that into your finances, that's fine... But now you're under pressure to go to war. In fact, you know it's the right thing to do, you'd be putting your nation at danger by not responding. But war is expensive. So are you going to push ahead on your tax reduction, knowing that you'll be sending an ill-equipped under-funded military out to fight? Or do you put aside your tax-reduction plan and get them what they need. Lives... Or money?

And this is assuming that everyone else is playing nice. Here's another one; your're clearly going to win, so while they're still in charge they put in place a bunch of popular but expensive policies to get people on their side. If you win anyway, your tax-reduction plan is no longer feasible. You may not legally be able to undo their policies, and even if you can, that makes you the asshole who eliminated the "food for orphans and veterans" bill to push through your pet agenda.

So making it punishable not to follow through on your plans wouldn't work, because it would put politicians in the position of choosing to do what's best at the time, or avoiding punishment by honouring old promises that aren't in anyone's best interests anymore.

Does this get abused to allow people to make promises they have no plan of following through on, or even know how to do? Hell, yes, but we can't solve it by forcing them to. My suggestion? Force politicians to put up implementation plans on how to make things happen. Show us where the money is coming from. Give projections of the impact of the plan. Have them cite sources for their claims. Make those things mandatory, with punishment for not following them.