Why do candle sizes have to be divisible by 24h?

There are various reasons: first, due to the nature of the 24h day period. Second, It's a matter of consistency. when we start a run it will be at a fixed time like hour candles will be on the hour, or 30mins candles on the first half/full hour. As a result, in 24 hrs you are trading on the same candle. 

We want to keep aligned with the way exchanges calculate and avoid being out of sync.

Example: If you use 7-hour candles, you will have three 7 hour candles and then one 3 hour candle, but it's displayed like the 3-hour candle has the same weight as the 7-hour candle which doesn't make sense from a calculation POV.