The rationale is simple: 

With David, Van Halen recorded more than a dozen cover songs on their six albums. 

With Sammy, they wrote more original music. In fact, I'm pretty sure it was all original. No covers. 

With David, they were pure party rock, which isn't a bad thing, but I doubt it would've held up much longer in a changing music industry. 

With Sammy, they branched out musically and tried new sounds and incorporating new instruments. 

With David, there was a lot of infighting that led to a parting of ways.

With Sammy... Oh wait, the same crap happened.

As a solo artist, David created one of my all-time favorite rock albums in Skyscraper (1988).

I listened to this album, along with Eat 'Em and Smile (1986) and A Little Ain't Enough (1991), a bit over the course of the last couple of days and they are just so incredible to hear. David is having fun. The band is really tight. All the songs are firing on all cylinders. Rock, jazz, speed metal. They're all great.

But, most importantly, as a solo artist, David brought to the limelight the impossible greatness of Steve Vai. 

Sure, Vai actually got his start as a guitarist and transcriber with Frank Zappa but his true emergence was with David Lee Roth's solo band. And then as his own solo act releasing killer albums like Passion & Warfare. 

Speaking of P&W, did you knew that this year is that album's 25th anniversary? In honor of the anniversary, Vai re-mastered and re-released it back in June in conjunction with a new album titled Modern Primitive. I haven't heard it yet -- it's queued up in Spotify -- but I'm really looking forward to hearing it. Vai is also playing P&W start to finish in a series of concerts. That would be all kinds of cool to see live. 

But, c'mon... 25 years? Seriously?