Moreso is actually incorrect, but it's one of those mistakes that people make so often that it has become standard. It should actually be "more so".
"More so" simply means "even more", and is similar to the Chinese "比...還要". We use to to emphasize that an already (adj) situation has become even more (adj):
-James liked to eat fish and after he moved to the beach he liked it even more so.
We often say "more so because" or "more so as" to explain why a situation is notable:
-Jane always sings well, and she sang very well last night, which is impressive more so because she had a flu.
-John already wanted to leave the party early, but he wanted to leave even more so as he had to be back in time to watch football.
In these sentences, "all the more so" can be used for even more emphasis.