Why are 90% of all my incoming emails going to All Mail and not to my Inbox?
In Gmail, all of your messages always go to "All Mail", hence its name.
What you seem to be experiencing is that some of your incoming messages are skipping the Inbox.
What is most likely happening is that you have a filter that includes the action "Skip the Inbox (Archive it)". "Archive" is just a special action which removes the "inbox" label from messages. (The "inbox" label is a special label, but it's still just a label.)
So the simplest thing would be to check your filters to see what's moving your messages.
There could be some other things going on, but you don't give enough information in your question to do anything but guess.
I was having the same problem and finally discovered what was happening. I get a newsletter that would keep going into All Mail and not the inbox. It was being classified as a "Promotion" by Gmail and appeared under the Promotions tab. Go to the gear icon at the top right, select Configure Inbox from the drop down, then uncheck all boxes except Primary. That will force everything into your inbox.