Simple
Tube amplifier electronic circuit is simpler when compare with commercial build microelectronic amplifier. It can be build by hard wired, no printed circuit board (PCB) involve.
High Success Rate
The tube amp components are much more rugged then solid-state. Most of the parts can withstand certain amount of stress without damage. Hence giving the greatest chance of success to the builder and save a lot on replacing damaged parts.
Quality
You can have full control of the amplification stability, reliability of the amp parts , quality of signal amplification and the best thing is the cost of construction.