I guess it's a trade-off between weight, price and volume.
The RB-112 is 26 lbs, 100 watts and $375 USD.
The RB-115 is 33 lbs, 250 watts with extension speaker and $575 USD.
The RB-210 is 38 lbs, 500 watts with extension speaker and $900 USD.
As Gramski notes above another consideration is bulk. It's probably good to be aware that the RB-115 is only 100 watts without the extension speaker. So if portability is important, and you only want to haul one piece of gear, then it would be more fair to compare the RB-115 at 100 watts to the RB-210 at 250 watts without extension cabs.