Minecraft "works" over TCP but I wouldn't call it great. TCP works great for things like block data since keeping those ordered makes it easy to send changes instead of resending whole chunks. On the other hand, entity movement being TCP is a real pain due to the head of line blocking which is one of the reasons PvP can be a pain and why it can be impossible to melee mobs without them hitting you first.
While agar is a casual clicker the core game experience revolves around sorting out collision conflicts. I can see how high performance would be desirable.
[1] http://wiki.vg/Protocol