I guess what's weird is the idea that you could split two coins and the two coins price is greater than the original. That doesn't make any sense even though it happened with Bitcoin cash.
The uncertainty keeps the price down. People expect a fine, but not how large it will be. I believe there are even cases where a company got the max possible fine and still their stock price went up.
Pre fork BTC price + risk premium ~ BTC post fork + BCH
Pretend like the fork actually taking place and working successfully as comparable to a company's earning reports. The second that news comes in, it prices itself in almost immediately.
However a rational (impartial?) actor should be willing to take that risk if the premium is big enough. That could mean buying pre-fork or selling pre-fork if there is an obvious opportunity. Which means in an efficient market the risk premium should be zero "on average". It's hard to reason about averages with one sample but that sample was distinctly not zero!
You could view Bitcoin + altcoins = total crypto market cap the same way..
You could also argue that if Bitcoin didn't fork previously, the total price of BTC alone would be higher than both forks now. Who knows?