BIG BOYS GUNS

[Sunray]

"...L7 Ordnance..." Chieftain did and the Leopard 1 does. Challenger 2's do not. They have a rifled 120. Either way, Point Blank Range on an L7 is 1,000 yards.
The M1A2 Abrams is mostly about its armour. That being the Chobham armour developed by the Brits. It also uses a 120mm smooth bore gun. It's downside is its weight(71.2 tons) and size(Hull length: 26.02 ft. Width is 12 feet.) It needs to be able to use existing European bridges.
The real issue with tanks of any flavour is their cost. Each one of those Abrams runs $6.21 million USD each and are estimated in 2016 dollars at $8.92 million US each. That being Hillier's reasoning even though it wasn't mentioned. The CF doesn't have the budget, the political will or the means of transporting tanks anywhere anyway.
"...the LEOPARD on this list..." Usually more about who created the list than anything else.