Educated guess: For added security, similar to how freshly mined blocks can't be spent until they are 100 blocks deep.
Keep in mind that Bitcoin's security model is largely probabilistic, meaning that while 6 confirmations are fairly safe, there's still a slight chance of a successful (and costly) double-spend attack. Or an accidental fork, even.
Now if a handful transactions get lost or double-spent, that would be already bad enough. Now imagine the same happening with either freshly mined blocks or a sidechain settlement -- an incredibly large amount of transactions would suddenly be invalid and a significant amount of economical activity would be lost. It follows that a generous safety margin is prudent.
There might be other reasons as well though. I haven't looked all that deeply into sidechains to be honest.
Doesn't that added security cost too much?
I mean that costs almost a day every time we transfer assets。