Apparently they got less revenue than expected because soda sales fell more than expected. That's a huge success imo
You seem to be under the assumption that they are trying to make people healthy. They are doing this for pre-k and if it flops because of less consumption, then this program fails for education and hurts the exact people they are trying to help.
*Edit* - just to add, this most likely hasn't cut consumption. Just causes more people to drive further and use more gas. Which last I checked is not good for our environment.
To be honest, it doesn't matter too much to me if they actually care about the health of people (which I think they do, at least to some degree), as long as what they are doing could potentially improve health. If the reduced consumption means they don't get enough revenue from that tax to meet its goals, which in turn leads to worse public services, then that is a problem, but that doesn't mean that the soda tax itself is a problem. If they removed it, they would get no additional revenue. It also has the potential beneficial side effect of better health, which is good for both the people and the economy.
Now, as I've said in this thread before, it would have a much bigger effect if it was a statewide (or even nationwide) tax, but baby steps... Slowly, more and more places are imposing a sugary soft beverage tax.