Because you may notice something in the texture related to the model that may not be working correctly as you thought it would, this would imply changing the topology of the model which implies changing the rig or skinning which is why one of the reasons it is a good practice to make sure the main texture base is working well before rigging is started. Or the rigging could be started in parallel but as a general setup (no skinning) while the main texture (not necessary the extra fine details) is being finalized.