Why not though? Do you have arguments against them having Chimeras or are you just saying that because the option has been left out for so long it might as well stay that way?

I disagree that Valkyries should be considered the iconic transport for Storm Troopers or that the list designers should forcibly restrict them in this fashion. Storm Troopers have always been riding Chimeras, since at least as far back as the 2nd edition Imperial Guard Codex in 1996. The Valkyrie was only invented around 2002 and it was never intended to replace the Chimera, but to provide another option, either of which may be used depending on preference and circumstance.

One could justifiably argue the most iconic transport for Black Templars is a Land Raider Crusader, yet that didn’t prevent the Black Templars list from being able to also still field Rhinos, Razorbacks and regular Land Raiders.

The background from the Armageddon Codex states on the Steel Legions that "a far higher proportion than normal are Mechanised Infantry, and it's not uncommon for over 90% of a regiment to be made up of such units.” I think in a list purportedly themed around the Steel Legion it is particularly ironic and unfortunate that the mechanised option for the unit was left out.

Well, it's a bit old but that doesn't matter IMO. Are there any news allowing ST to use chimeras? IMO it is an well-suitet option.

The missing option of ST in Chimera's is more an oversight IMO, rather than excluded for whatever game balance or fluff reason. That the Cadians can currently get ST's in Chimera's doesnt exclude that the Steel Legion can't have them.
Its also cheaper for the new player to acquire Chimera's, or good proxies, rather than via Valks. And who knows how long Aeronautica will last in the FW catalogue...
The Cadian list is still different enough to have its own flava if you give the SL list the ST's in Chimera's. Also provides some spiffy paint scheme's for the Chimera's, ST wouldnt like to have the same coloured transports as the normal grunts!

Tee

Which Cadian list are you using? Chroma's list (v1.5.2 is the latest I have) has not been in active development for at least 2-3 years, IIRC. The version in the army compendium (v1.3) is the list that is being most actively developed. You can also find a link in my sig.

Back on topic, the Steel Legion list is already heavily tilted to a mechanized flavor, so there will usually be at least a couple formations in Chimaeras before Storm Troopers are added. STs were given Valks to better distinguish them from regular troops and emphasise the elite nature of their formations. There is no fundamental reason they could not be in Chimaeras, but you'd end up with a formation that would stomp all over the toes of the Mech Inf Coy. 3/4 the points, nearly as good in engage, better AT, and about 75% the AP. Oh, and they'd be able to garrison. Better to make them different enough to fulfill different roles.