va-diim: Because "być" is not predicative, as "Jakie" is not subject of the sentence. Not everything that comes after "być" goes to instrumental; only predicatives do. E.g.: Jestem tutaj (I am here). Być tutaj. Być wesoły = to be happy, Być w domu = to be at home, etc.
Jakimi is a totally different thing. It would be clearly question about the properties of something, but posing such a question correctly is not evident, often unnatural and sometimes is quite difficult. It is not a typical question, it may be posed while asking about someone's preferences. eg.
- Jakimi dziewczętami się interesujesz? [What (sort of) girls are you interested in?]
- Jakimi samochodami lubisz jeździć? [What (type of) cars do you like to drive?]
or in singular:
- Jakim on jest mężem? - [What (kind of) husband is he?]
- Jaką mnie wolisz? [What do you prefer me like?] - (clearly a woman asking her closely related man, and it may concern eg. the colour of hair dye, dress, attitude and a whole range of other things including intime behaviour) - therefore it is often necessary to narrow the question down, eg. Jaką mnie wolisz, w rudym czy blond? [What do you prefer me like, in red or in blonde? (showing off with her hair dyed)]