Prof2DMA
One possible reason behind the canard layout is the opportunity it offers to land and take off using a flapped delta. On the Tu144 as soon as the very high lift canards were extended the very large nose up trim change allowed (required) the wing trailing edge surfaces to all be dumped a long way down, doing great things for the wing Cl max. Compare this to the Concorde which unsticks and flares with a negative flap angle on the wing. I believe the difference between approach speeds with the two Tu144 variants (with and without canards) may have been as much as 30 -35 kts.
JF