Companies often present crew with a pre-computed flight plan. Alternate in many cases is a lesser standard airfield, if possible not too far from destination. The computer flight plan is based on no enroute delays (maybe 5%) to that alternate. Well, excuse me, but get a dozen aircraft wanting to leave the holding pattern at the same time for the same alternate, and the flight plan is simply a wish list.
Congestion at the alternate(s) uses up heaps of fuel, due
runway backtracking by previous aircraft
controller not up to the multiple arrival tasks
procedural approaches without radar
etc
The gradual realisation that many factors are conspiring to make continuing flight less attractive puts pressure on crews to land under unfavourable circumstances. Operators sometimes put pressure on us to carry fuel close to the flight plan, with little consideration for full possibilities at destination and beyond. Many captains resist this pressure. Some do not.