Gas pipe has a thicker wall, and is more rigid....less flex especially if you have heaver tables, work pieces or cutting loads..
The thicker the wall, less flex, more weight and cost..
The harder the material, less flex, more cost [usually]
The larger the diameter, less flex, more cost [and larger associated parts]
It's a lot of tradeoff compromises, gas pipe is pretty cheap, easily available, and reasonably rigid...
larger diameter , harder pipe with thicker wall sizes [like seamless tube], will give much more rigidity, but at the expense of larger support items such as carriage parts, supports, bearings, etc..
the most versatile and reasonably cheap is steel , as opposed to harder aluminum alloys, carbon fibre, trussed wooden composites..
best choice [imho] is plain old cast iron, and lots of it...but not ideally suited to a hobby grade machine, prettiest is aluminum..
basically I feel that conduit would have too much flex, and the time involved to support it better, or to go to a noticably larger diameter isn't worth the bother as the cost between gas pipe and conduit isn't that much..