Because the block changed. We he finds one, its a whole new block by then. All his work on that last block will never be given credit.
There is no "partial work" on blocks. Each random hash is either a valid share or not. When you get notified there is a new block, you keep trying hashes to see if they meet the new block instead of the old block. The only time a new block has any effect is if you report a share moment too late and it is 'stale'. But that's true for anyone.
The probability of finding a share is independent of your miner's speed or how many blocks you've looked at previously. A new block notification has no effect on this (unless the difficulty for the new block has changed).
Meanwhile, fast miner got 9 on the last block (or 8, or 10, whatever you like). Even if slow miner finds a block this time, fast miner probably got 8-12 shares again.
If the fast miner is 9x the speed of the slow miner, they should be reporting shares 9x as often as the slow miner. If it takes about 5 shares to find a block, then for every 2 blocks found on average that is 10 total shares, 1 share from the slow miner and 9 shares from the fast miner. So every other block the slow miner didn't report a share during that block. But that doesn't matter. The slow miner is reporting 1/10 of the total shares and will earn 1/10 of the total payout.
Over time, the block changing hurts the slow miner more.
Untrue.