Because I can see at least a couple of potential reasons why raising the percentage of elderly patients may not be as simple a solution as you suggest and some other aspects.
1) A large number of elderly patients have mutiple medical issues and be on several medications. Having high % of them may have complicated assessment of the efficiency and particularly of adverse reactions, especially in early rounds.
2) The population of people in the older age groups willing to volunteer (or get mdeical approval to do so) might have been lower than younger ages
3) This was an untested vaccine, having large numbers of the most vulnerable take it seems like a high risk/reward situation that may well not pay off, especially for the participants
4) I have no idea, and I would be interested if you do, if there has been documented large differences in the reactions of old people versus young to previous vaccines. There has been a history of lack of gender representation in other medical trials causing problems (gender differences
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4800017/) and some diseases impact racial groups differently (eg sickle cell) but when I was more seeped in this stuff earloer in the year I don’t recall anything re age although I could have missed it/forgotten.
5) I assume medical statistical analysis is quite developed in these trials and even with relatively small data sets they can make some reasoned assumptions, at least to decent confidence intervals
6) The vaccine has not yet been approved and Phase 3 results have not been released as far as I am aware so declarative statements such as “the Oxford vaccine is not what we need” seem premature at best.
But I have no medical training - I have no idea how big/small some of these factors would be so given a choice between you, me or the scientists I’m betting in the latter to know what they are talking about!
However an olive branch - show me a medical paper, or even a decent twitter thrread or scientific article, that summarizes your concerns and I promise to read it and get back to you with something more substantive.