I also do not understand why. When they were the first, they could have made some money by making it available to top professional players, who probably would want to pay thousands of dollars to play against it. Now, there are enough reimplementations that are on the same level or maybe even beter than AlphaZero. I guess that could be one reason why Google is not making it available, because it would be clear that they are not as good as others. At least they would have to invest money in keeping a server running. Anyway, it is clear that for DeepMind Go was but one stepping stone in developing AIs. They have moved on to other games, games that do not have complete information (as with board games like Go) or that involve guessing behaviour of others (like Poker).