How Do You Pronounce 'Diabetes?'

I wonder why people pronounce words differently? All my life I pronounced diabetes "diabeetus," not "diabeeteeze," so every five minutes or so, when I hear a commercial on the radio or on the TV I hear it pronounced the "ease" way, not the "us" way, and it always kind of jars me, so I finally decided to research it. This was driving me crazy, so I googled it. I was directed to a 2001 commercial for Liberty Mutual with William Brimley, and this guy pronounces it the way I pronounce it. So, it could be a generational thing. I do not know if this is because my mother was English or if this is the way people pronounced it in the 50s and 60s or, well, always, until recently. But diabetes is not the only difference. How do you pronounce "often?" I always pronounced it without saying the "t" sound, so, again, I hear that word on the radio or on TV multiple times a day, and I have to ask myself, again, why. I know, most people could not care less about things like this. Also, most people seem not to care about how anything is spelled either, and this always makes me wonder as well. I told someone today I was a "recovering grammar Nazi," and I suppose that is what I am. Although I don't know if I'm recovering or not. I have also notice that people don't pronounce "t"s anymore in words like "kitten," and "important." Why is this? How does this happen? I know there is a word for that. It's called "t-glottilization." Examples: button, mountain, kitten, as you can see, this usually happens before an "n." I first noticed this tendency 11 years ago, when I came to work where I work now, where the teachers are mostly Millennials, born 1981 – 1996, I think. I thought it was so strange, at first, until I realized it must be a generational thing. I just think that this is interesting, just one more thing to make people like me wonder.