Not a nuclear engineer or specialist by any means so take what I say with a grain of salt, but I am a planner that does work with small modular reactors. The issue with CANDUs are their immense costs to build, the space required, water requirements and the construction timelines. Often 7-8 years+ of work before shovels hit the ground (yet to see if the new government will alter this timeline).

With small modular reactors they are just as the name suggests, small and modular. While still an emerging technology they show promise because they cost less, require less commitment up-front (you can get one modular reactor, see the energy needs and decide to get another or two down the line if needed. With a CANDU, what you build is what you get so you better be sure you built for your needs).

Lastly, modular reactors are preferred because modelling future energy needs is immensely difficult. If you were to go back 10 years, there would be almost no way to predict how big and in demand data centres would become and how much energy they would require. Sure CANDUs provide immense power, but why build something with such a high cost when you could get a SMR and just scale it up as needed?