From bowiki

In traditionally built drums, a bar spanning the open back of the drum was common.

This is for a number of reasons

• The player could hold the drum
• The player could use the bar for leverage when playing with their hand on the skin.
• The Solid Shell wooden frame is prone to warping and the crossbar provided stability.

Where have the crossbars gone?

With modern lamination techniques, the drum shells are very strong without the crossbar and therefore it is are no longer required for structural support purposes.

Modern Playing styles on modern drums with thinner skins do not generally require the pressure-assistance of a bar for the left hand

With practise a non-crossbar drum can be held by the player when standing. It may limit full expression however