I am not sure why they don't add Coinmarketcap to the list, but I would like to use the price source from Coingecko or Coinmarketcap, if they add it in future.
Here's why:
Coinmarketcap's free/paid API requires a registered API key per user with limited calls per month and very low hourly limit.
While it's possible to set an API key for Electrum clients to use, the limited API calls and hourly limit will not be sufficient for its userbase.
You have many options in the drop-down list for Fiat Source and if you have problem if fiat value display with one source, switch to another or other sources, before your problem is solved.
Actually, he can only use 5 from that list since he's using "
Download historical rates" setting.