St. John's College, Oxford should be the most popular?

You are perhaps unaware that every college in Oxford has a library, that several have cafes in addition to Halls, that several provide accommodation for all years of the undergraduate degree, and that several provide book grants of varying sizes.
It's not a good idea to fixate on any one college in Oxford, or on Oxford in general. Each applicant to Oxford is, on the numbers, more likely to be rejected than to be offered a place, and about one in four offers is made by a college to which the recipient of the offer did not apply.

It would be a right pain if everyone applied to the same college as it would just mean almost everyone would have to be reallocated.

New the library and study centre may well be, but it is also ferociously ugly. That these eyesores cropping up all over the historic gem of a city are allowed beggars belief. Only in UK…

Since St. John's College, Oxford has the cheapest food and drink, it's own cafe and 3 or 4 years of accommodation at the college. So, why does it not attract the most applicants? Moreover, they have a massive new study centre/library completed in 2019 and also give £400 book grants every term?!

It does get the most applicants, or maybe the 2nd or 3rd most. Over 1000 people apply every year

Yes, you're right. Because I was told by the admissions officer that 80 to 100 are Law applicants each year!!! So, competition is 'fierce' for Law places at St. John's College, Oxford.

It is worth reiterating that college choice is not as important at Oxford as it might be at Cambridge. In the 2024/25 admissions round, over 40% of those who obtained offers to study law at Oxford received their offer from a college to which they had not applied. Each college at Oxford has the same admission requirements and each makes the same offer (AAA or equivalent).

People take a fancy to colleges for all sorts of reasons. For some people it will be the lake at Worcester or the gardens at LMH, or the fact they got to see the college tortoise at Univ. Or maybe they really liked the student giving the tour on the open day at Hilda’s or someone from their school went to Oriel last year and is having a great time. Some people focus pragmatic things like accommodation costs and library opening hours, others go on the basis of their gut feel. There isn’t a right or wrong reason to feel a preference for a particular college. St John’s has never particularly floated my boat but other people love it.
It would be a right pain if everyone applied to the same college as it would just mean almost everyone would have to be reallocated.

I think that Queen's College may have the most beautiful college library in Oxford. The Upper Reading Room of the Radcliffe Camera is, I think, the most beautiful of the university's libraries, but the Taylorian is also lovely. In Oxford, you are never short of beautiful book-filled rooms to work in.

Noooo Lincoln college library looks the best from the pictures I've seen