Because Ole wanted to play out from the back and figured that Smalling wasn’t up to it. The debate around his quality was always related to his (exaggerated) limitations on the ball, not his defending.
So ultimately he overall wasn't good enough.
If he was a very good defender I think his clear deficiencies with the ball at his feet
might have been overlooked. The fact is he was decent at best, one of United's better CBs defending because he's good aerially and also pretty quick, but always felt like he lacked composure and had a mistake in him.
This is exactly the same situation with Evans when he left, and Blind to an extent to. United fans pining over past CBs when there's were out of form because they were now playing pretty well at other clubs. Ultimately like I said they weren't good enough for United's level and that's why they were binned.
I just find it strange that considering you seem to have a pretty good partnership at the moment (small sample size albeit) in Varane and Martinez, that this sort of thing still goes on.
Most of your post I agreed with but the bolded makes no sense at all. International football is about tournaments not long grinding campaigns and so all you need to consider is who is in form and who fits your system. Smalling has been in great form for over 12 months now and is better than any of the current CB options in the squad and plays in an identical system to the national team where he is unarguably the lynchpin of the Roma defence. The only calculation for Sir Chinny Waistcoat to consider is will England be more likely to win with or without Smalling, the answer is clearly with Smalling but thankfully for Wales the England Manager is as good at picking players as he is at taking penalties.
I mean this isn't necessary true, international football is largely about cohesion as well. You still have to build a squad, that's what the successful teams over our generation like Germany for example have done. These are players that don't get to train together regularly and have to build chemistry just like any other side, that is obviously going to take a bit longer than you'd see at club level.
Smalling hasn't been in the England setup in 5 years and hasn't even trained with half of the players who'd be around him as a result. I wouldn't want to draft somebody in in that type of situation to then immediately go into a World Cup.
You can argue whether he should have been in and around the England setup the last year or so, that's entirely different than bringing him in now. I would still say no personally, I don't think he's been as good as people suggest he has been for Roma, I think he's been decent but not good enough to force his way back into the England setup all things considered.