With the weather as it is, it is hard enough getting housing and fodder together for the mainstream business! Yes milk prices are low per litre but dairy cattle produce 7000 litres per year, or more - so perhaps £1600-£2000 revenue per cow per year. The best suckler cattle can't compete with that - so rearing Jersey bull calves simply isn't in the same league.
On a dairy farm, all waste management is likely to be centred around slurry; calves are best reared on straw, which means a different kind of muck, different systems, capital equipment, handling facilities, etc, etc.
So whilst it suits some dairy farms to rear their own bull calves, and heifer replacements too, you can't expect them all to do it.
You can now get sexed semen, which a lot of the dairy farmers have tried - but it seems to be less effective than the unadulterated stuff. Plus you always need a bull to follow on, and it's not 100% anyway, so there will always be dairy bull calves.