How come I don't get the preferred seat of my choice!!?

How come I don't get the preferred seat of my choice!!?

• The number of seats available for pre-assignment is limited. There is a growing trend on this too.
• Bulkhead seats are always reserved. In many cases, those seats are reserved for families traveling with babies, or other physically challenged travelers.
• Airlines change equipment scheduled. They will cancel all seat pre-assignment. That had happened to me couples of time!
• Each airline has a specific policy regarding check in time limits on prearranged seats. If you do not check in within certain time limits, you may forfeit your preferred seat
• You may be required to pay a pre-seating fee for the privilege of selecting a preferred seat. Tango of Air Canada charges $10.00 for preassigned seating. Air Transat charges a advance seat selection charge too.