If you were able to set depth to more than 100, that was definitely a bug. Not being able to do it is the correct behaviour.
I don't think there is any particular reason for the limit being at 100, it seems like it was just set as a sensible limit. Obviously it can not be infinite as that could eventually lead to running out of memory.
The thing with the depth property is that each additional increment is essentially adding a whole extra sheet. If you are using relatively small sheets, let's say 10x10 for example, that is fine. In that case each depth increment would be adding an extra sheet with 10 rows and 10 columns, which would add up to a total of 100 new cells. If your sheets are larger though, each increment could potentially be incrementing the total cell count dramatically.
The limit for width and height is currently set at 1000, in the case of using a sheet of 1000x1000, each depth increment would be a million extra cells, which could rack up memory pretty quickly if left unlimited.