First let me say this isn't a thread to take a shot at Mozzy. this is more of an overall question. Over the last few years we seem to be good or great at drafting almost every other position but for some reason we just can't figure out defensive tackle Other than osa. Who else was a good to Great defensive tackle that we have drafted over the last few years.
It's primarily where we draft them. We're expecting starters in the third round and beyond. Look at Philly: Milton Williams, a third-round pick, is a decent player, but the Eagles haven't really relied on him to carry the position. Instead, they've primarily spent early to mid-first-round picks on the starters (Fletcher Cox-now retired, Jordan Davis, Jalen Carter).
We've spent one first-round pick on the position since 2005. Players taken in the fifth-seventh rounds (Quinton Bohanna, John Ridgeway, Justin Rogers) are always long shots to make an impact. I don't know how Mazi will turn out, and I definitely don't think Dallas is great at drafting DTs, but when you spend one first-round pick on the position in almost 20 years, this is what you're usually going to get unless you are lucky.
Since Parcells left, we've drafted five DTs in the first through third rounds. Parcells drafted five in two drafts (2005 and 2006), but he also was switching to the 3-4 so he needed the right kind of players for that. He also brought in NT Jason Ferguson, so he essentially completely rebuilt the line those two years (Spears, Canty, Ratliff, Ferguson, Hatcher and UDFA Stephen Bowen).