Why the low floor recommendation for the Valley line?

[Myrtonos]

Although I am an outsider, I wonder why there is a low floor recommendation for the Valley line?

It seems that the Valley line will be separate from others because of this. Low floor will mean fixed bogie vehicles with short sections and wheelboxes where the bogies are and control equipment in the roof.

The existing lines also have level boarding at all stations but with higher platforms the existing rolling stock has pivoting bogies, a bogie under each articulation and control equipment under the floor.
They also have higher capacity for a given train length.

Also, the new vehicles for the new line will come from a supplier, Bombardier, which still makes high floor LRVs.

Furthermore, going with a separate platform/floor height for just one new route will reduce opportunities for a large order. And it won't be possible to transfer rolling stock between different lines. Cities with smaller systems, this includes newcomers, are offered off-the-shelf designs.
Cities with large fleets and renewing them, such as Toronto, tend to make large enough orders that there are multiple bids for custom built vehicles.

Splitting the fleet into different standards compadible with different infrastructure would reduce the ability to get that kind of economics of scale.