Why FSB?

The reason Photoshop requires PSB is if the file size is too large (over 4GB) for PSD to handle. Your document specs are not the direct cause of the problem; that combination of bit depth and pixel dimensions is normally not going to exceed 4GB on its own. The width and height in pixels isn’t very large by today’s standards, so it isn’t that.

What is probably happening is that the nature of the 19 layers is what makes it exceed 4GB. The layers do multiply the file size beyond would it would be if it only had only layer, and if they are being used as embedded Smart Objects then their effect on file size is larger. This is mostly about pixel layers, because other kinds of layers such as type, vector shapes, adjustment layers, and fill layers (solid color, gradient, pattern) don’t need very much space to store.

The bit depth might indirectly contribute to this, because for example the file would probably be only half as large if saved at 8 bits per channel instead of 16. So the cause is probably a combination of more than one of those factors.