My thoughts in no particular order:
a) Probably not too many pilots type-rated on Q400 (purely a guess).
b) Airlines are run by accountants who like the cheapest option and quite a few of these schemes will take on cadets on a reduced FO salary.
c) Airlines are very conservative and the schemes provide a known quantity 'product' with pre-selection and monitoring throughout their training without any commitment to actually recruit anyone who turns out to be a donkey (based on skills or more likely personality / work ethic etc) so it's a bit like a year long interview process taking place during the training.
d) Most airlines don't recruit captains but would like to take pre-selected FO's who would be suitable for LHS are are likely to be loyal enough to hang around long enough to make the move.
Probably other reasons but that is some of why I think these schemes form part of the recruitment these companies conduct.
DISCLAIMER: These are only possible explanations and not my personal beliefs - so no requirement for flaming .... muchas.