I think it's beneficial in two ways to add new series, not just for Smash but for Nintendo.
1) It exposes people who wouldn't be interested in them to try them out and learn more about them. If newer series weren't added, such as Xenoblade or Punch-Out!!, then it doesn't give fans a chance to actually interact with the series, or even further, Nintendo and gaming history.
2) Nintendo gets to capitalize on it and make more money. Fire Emblem is the prime example of Smash as a vehicle for marketing. Fire Emblem wasn't really on the minds of anybody in the West around the time of Melee, and it's now one of the most profitable series for Nintendo and is growing.
More than anything, I think Sakurai literally is just looking for what's the most fun or interesting, and new. If he decided to stick to stuff that sold then...Smash would probably get stale.
If the newcomers from different series are boring to you, I'd highly recommend you play some of their games. They're excellent.