Planning restrictions perhaps in Arsenal's case? Maybe 60k was the most the local council and police thought that the roads and transport infrastructure could deal with in a heavily built up area of London?That makes me wonder why the Arse only built the Emirates at 60K then.
Suppose it could be cost / profit analysis for extending it to another tier or does that have an effect on any build?
Debate here about whether they can expand their stadium or not:
http://www.not606.com/threads/emirates-stadium-expansions.263569/
Would be pretty short sighted if they an't. I imagine they can in some way. I remember people saying we couldn't expand ours without taking the roof off all tiers and here we are!
The suggestion that stadium couldn't be extended without a new roof has in fact been proven by the design of the South Stand. What most on here were saying is that the CableNet system couldn't be removed with a building a whole new roof. The extended stand is designed around the CableNet in fact it's extended as the new roof is supported by it.
The new roof isn't supported by the old cable net, it's completely independent from it and is held up only by the two new large supporting structures. When they eventually do all 4 sides they'll be able to take the old cable net away.