No particular order, and obviously some of this may not apply to your school:
Not educationally necessary - it's a 'celebration' not a formal learning goal
No funds - first 1% of pay rise this year must come from school budget
Staffing - trips generally need extra supervision/staff
Overburden of risk assessment - admin disproportionate to small activities
Paid-entry places have become very expensive - local park is free!
Behaviour of a minority of students has become a lot worse - supervision
Parent's non-essential special requests - 'he/she won't eat this or that'
Some parents won't pay - either no money, or won't subsidise others
Fewer staff willing to go the extra mile - had enough already!
Fewer parents willing to volunteer to help for a range of reasons
etc etc...
Thing is, OP, an 'end of year' trip is a relatively new thing over the last decade or two. We didn't have them in the past. Yes, of course we had mid-term, 'paid-for' trips, which were for learning - but we didn't have 'celebrations'.
Unfortunately school trips, although a lovely luxury for some, will always exclude poorer kids, or those with additional needs - unless, as a PP has said, there are some reasonable cheap options made available, too.
A fairer solution would be for the school to fund-raise for the trip throughout the year...but if it's anything like our local schools there is already lots of fund-raising for essentials like books! So no money for luxuries.
I do wish there was a fair way that richer parents could be incentivised to pay a little bit more, though ...Perhaps we need an SUV charge at school pick-up 😂