Why is the pneumonia vaccine only recommended to seniors? wouldn't it also benefit all younger adults, esp. considering how severe this flu season is?

Not quite true: We have 2 types of pneumococcal pneumonia vaccines in the US. Babies and toddlers are routinely given pneumonia vaccines. Anyone who has bad lung disease, or has a weakened immune system, may get a pneumonia vaccine. Normal, healthy, non-smoking, young or middle-aged adults may not benefit from a pneumonia vaccine if the vaccine doesn't boost their immunity to pneumococcus by a noticeable amount.

Duration of effect: All bacterial vaccines like that for pneumococcus, whooping cough, etc. Have a limited duration of effect. It is given in infancy & covers the vulnerable early childhood years where risk is high. It is offered to any group with a weakened immune system, and more recently to seniors. In less vulnerable populations (most adults) it is an option but with marginal benefit.