In the UK, it's because it's the defence's job to notice this kind of thing, but paying for a good lawyer is life-ruiningly expensive, and public defenders via legal aid are paid hourly, but have a cap on the number of hours that's never enough to deal with anything properly, so are either rushing everything and miss things, or are massively overworked and earning well below minimum wage when their unpaid work is included.

I don't know if the same problems extend to Canada, but it's fairly likely, as the adversarial approach just inherently means the defence has to do loads of things that would be someone else's job or not need doing at all under alternative systems, so you have to have a trained professional doing lots of work, and that's inherently expensive, but taxpayers don't like spending money on defending people who probably did crimes.