You'd be better to attribute that to them being under-resourced to deliver software updates to all models of phones immediately. They focus on shipping to the high end phones first -- those customers have paid more for that support. Some low end devices just never get to the top of the priority list.
If you want upgrades, buy the best phone you can. If you are buying low end, you have to realise that it's essentially locked to the version that it ships with.
That's my take on it anyway. I don't have any inside information.