There can be many reasons, for example:
1. The case workers are being lazy and the IRCC is overwhelmed by its case backlog. (The most likely reason)
2. There are problems in your file.
3. They have to verify something with an uncooperative reference (company, government, UN, etc)
You should request GCMS notes to find out what is going on, if you are in Canada you can request it for free through the Privacy Act.
https://www.cbsa-asfc.gc.ca/agency-agence/reports-rapports/pia-efvp/atip-aiprp/req-dem-info-eng.html