I mis-spoke previously...
At the time - Jar was originally banned in sanctioned play
except type 1(restricted) and sealed(you can play with as many as you got) - which included block... This kept it out of the type 2, extended, and 1.5 arenas...
Now it is specifically banned in all but type 1 and sealed... (not eligible for type 2, so no ban is necessary)...
As for the tempest stuff being an issue - The jar deck that got it banned included the tempest speed...
Other banned block cards came much, much later - and mainly because of the free creatures'spells that blue had - such as great whale...
Think of the damage you could do with gaea's cradle, academy, serra's sanctum, etc... all in play while untapping lands for a free whale...
"Blue - bends the universe to it's will... Until I cap the blue mage with my 45..."
A note - academy also got a lot of it's strength from cheap artifacts - petal, diamond, monolith, etc... and even more in type 1...
without tempest's speed - urza's cards would not have been so broken...