The 74141 is an open collector output with Zener protection diodes. When the outputs are OFF... no current flows through the TTL IC gate output transistor though the zener protection diode will happily work independently of gate status. When the TTL Gate is ON, it supplies a ground path to the NIXIE segment, closing the circuit and allowing it to light. A standard neon lamp operates at 60-80V, a Nixie operates at 170V. using the same ttl gate IC for the two devices is impractical.