Because if we'd only offered them a few thousand unless they increased the 2,800 allocation, they would have realised that unless they agreed their fans would have been very disgruntled to say the least.
If man U got Celtic in the Uefa Cup (or Champions league if the rules are the same), they'd only need to give 5% of the ground which would be 3,500 tickets. Can you imagine several bodies including the manc police forcing Utd to give over a whole stand. i don't think so.
Your Town, Your Team, Celtics' Tickets.