You would be surprised to know that Gold can vote for 2/3 of the game options too (except for survival vs normal).
I like it this way to be honest, especially considering that Cubecraft as a company has been promoting Diamond a lot lately. It’s more accessible in terms of money, and if people want to upgrade to obsidian to support the server, well.. they always can! Obsidian has still more perks than Diamond both in terms of voting (mainly in other games) and in terms of cosmetics too - you get more stuff and cubelets as a whole. It’s just up to people to decide. Tower Defence rarely gets games with ranked players during certain hours of the day, so at least more people can vote for it in a way.
Plus, let’s put it this way. If you saw that diamond people can vote for double income (which is something that you can already do with gold actually) instead of obsidian, as a non ranked player would you be willing to spend 20 or so dollars/euros/pounds/your currency or would you prefer spending over 100? I know for a fact that from a marketing point of view, more people would want to buy something that gives the same perks at a cheaper price - and Diamond is way cheaper than Obsidian - so more players would be willing to spend money on lower ranks in order to get that perk, meaning the server would profit. (If you would like to learn more on the matter, that’s the consumer’s optimum in microeconomics)