I have a harder time understanding why any "solo miner" would pay 2% to some "service provider" rather than mine on its own.
I suspect that this is probably the biggest reason:
Solo.ckpool.org is extensively connected to high speed low latency bitcoin nodes for rapid block change notification and propagation.
Solving a block is only half the battle. First, the software that is building the block headers needs to hear about solved blocks as quickly as possible so you don't waste time mining a stale block at a height that is already propagated to a significant portion of the network. Then, once you do solve a block, you need to get it propagated to the majority of the network's hashpower before anyone else does the same.
2% a a fee feels a bit high to me. While there is some risk of losing a block due to poor propagation, I'd think that for most that risk would be less than 1%?
CKPool claims:
This is a NOT-FOR-PROFIT pool
But, I am left wondering where that 2% is going if it isn't being kept as profit. Perhaps running a pool like that is more expensive than I realize.