Yep, its your age of credit & utilization of that credit. You should never close accounts.
Just last week Discover said they were closing one of my oldest (but unused) cards. That would also be maybe 10 percent of my available credit. A simple phone call usually keeps it open and available.
Since I have a good many accounts - some of which have been open 7-8 years, I have a longish credit age, and just over 100K worth of limits I dont use...so if I have 2-3K on one card I'm only using 2% or so of my available credit. Plus per the reporting bodies....I have made hundreds of "on-time" payments, as they are just zero, but accounted for. This helps scores soar.
I also often go for credit limit raises whenever I can on these accounts.
Based on what you've done here you sound responsible enough...sometime there is no reason not to use a credit card daily. The cash back, miles, or whatever 'reward' you are after is much better than an old checking account debit card. Might as well get 2% of whatever you spend back or something.