USA is holding back tanks because IFVs (like the M2 Bradley, German Marder, or Swedish CV90) are more important for this upcoming fight.
M2 Bradley is an excellent vehicle. No, its not a tank, but its the vehicle the Ukrainians need today. With over 100+ M2 Bradley donations, and 90+ Strykers (a lighter vehicle that serves a similar troop-transport / mechanized infantry role), its clear to me that the USA cares more about practical battlefield issues rather than the somewhat political "Tank" question.
Based on the reports of Trench-warfare, masses of Russian infantry, and satellite photographs... we can see that the 25mm autocannon on an M2 Bradley with 900 rounds stored is going to be more useful than the 120mm cannon of an M1 Abrams with only 40 shots.
Furthermore: the Stryker vehicles with Mk 19 machine-grenade launchers are obviously better for clearing enemy trenches at long range than any other vehicle.
Yeah, tank blasts are ferocious, but not the best weapon against the huge network of trenches Russia has setup. Grenades, heavy Mortars, self-propelled artillery, and IFVs / M2 Bradleys are whats needed.