Why no tsuba?

"Because that's not the way we do it in this school."

Beyond that, I would note that a decent tsuba will cost as much as a bokuto and more than a bargain judo-style keiko-gi, and also note that aikido has had an orientation toward keeping entry-level equipment prices low for 60+ years. Materialist explanation often misses part of the story. That's why I started with the oft-heard quotation above.

Hope this helps.

Fred