Citi Strata Elite Denial - Odd Denial Reasoning

I have been a Discover Credit Card holder for almost 15 years, and they were recently bought out by Capital One. This got me curious to start looking for a new credit card home and I came across Citi as the one that best that suited my lifestyle.

Over the last two years I've averaged an 825 FICO score and low revolving utilization. I recently finished paying off my vehicle and have never missed a payment on any revolving account or mortgage account. I have a 125k income and have proven through my credit report that I'm financially responsible.

I decided to apply for the Citi Strata Elite but was denied. I got mail from Citi approximately a week later with the reasoning, which had me puzzled.

Per the letter, I was denied for "Your credit report shows too few established/open accounts." This was the only reason on the letter that I was denied the credit card. I called the reconsideration line and still wasn't able to get approved or help. I'm really confused on understanding what this actually means and if I messed up. The majority of my credit card spending goes on my Discover card, but I have three other open credit cards (B of A, Lowe's and Kohl's) and my mortgage. This gives me a total of five accounts.

I have not applied for a credit card in years and maybe that's why I don't fully understand the denial reason. Here's a few questions I have.

1. If my credit report says I have too few established/open accounts, which is five, how many is considered a good number?
2. Was applying for a credit card with a high annual fee too ambitious in the beginning? Should I have applied for example the Citi Double Cash first, a no annual fee credit card?
3. When should I try again on applying for a Citi credit card whether that's the Elite or a no annual fee card?

I appreciate you reading my post and hope to hear your thoughts!