Because it is "safer".
IMHO.
And aside from RTFM or anything else.
There is less likely to go wrong with Update sync vs. other methods.
A user then wants to use some other method, fine, let them.
But make them make that conscious effort to do so.
(As it is, I use Update.
I might use some other method, but I'd have to read & understand, very well, just what the implications are in using other methods [& I don't know, particularly].
The other day someone asked me... & I said - FreeFileSync.
And I threw it onto their computer.
And I said "Update - because it is safer".
And they said, well what about... & I said, "Update is safer".
And if you want to use some other method, then go about & read & learn - first.)
viewtopic.php?t=9914#p36692then by the same token, IMO, & in IMO even if that isn't the reasoning ;-), is that the default Sync method should be an Update sync, rather then Two way, simply cause, well, IMO, Update is "safer", in particular for a novice user.