Because a decaminute is fixed, while a block is variable.
Let's say it's dynamic, sounds even better. It adapts to a ten minute frame, but sometimes, when you like write a test in school or try to defuse a bomb you can have even more time, while home time might come earlier.
And that are just examples on how a block based time system would make the world a better place!
How do you think god created the word on seven days? There wasn't anything so blocks took a loooong time, you know? On the other hand he is god and therefor his computer can solve blocks like instantly, which explains why millenniums are nothing to him. See, it all makes sense?
Blocks are something completely natural and self-adjusting, just like free markets.
