The problem is that video replay only works for negating goals really, with regards to the offside rule.
Example 1. Someone scores and the linesman misses the offside. Play is challenged, overturned, goal doesn't count.
Example 2. Player is whistled offside on what would have been a clear 1 on 1 with goalie, perhaps he even slots the ball home before anyone knows the play has been whistled dead for offside. Even if the team challenges, and the play is overturned, you will not receive a penalty or be given a 1 on 1 with the keeper. At best you could hope for a free kick or a corner or throw-in.
So what happens if Team A, has a goal disallowed for offside in the game on replay evidence, but then also has human error blow play dead on two more through balls that would were either slotted home, or would have provided 1 on 1 vs keeper. Team A, perhaps loses because of this, 1-0. The human error on the goal that was originally allowed would have made up for two more disallowed chances on wrong offside calls.
There are many games where a team gets away with offside a time or two, and gets whistled for it other times. How do you account for the fact that one replay can negate a goal, and the other replay cannot offer the same quality of scoring chance if it turns out the linesman raised his flag wrongly. Until that conundrum is figured out, it's lame to institute video replay for offsides. I think it would result in much worse.