Rationale for titant standardization in alkalinity measurements [ISO 9963-1:1994]
According to this listing, $\ce{Na2CO3}$ is on of the primary standards used in volumetric analysis, i.e. is easily available in a reproducible form and concentration, has a high molecular mass, is -- once the container is opened -- not (so much) hygroscopic; and is stable.
I speculate, a dilute solution of HCl (0.1 mol/L), if stored over weeks in a bottle recurrently opened, maybe stored on a window bench (seen so frequently in the undergrad's labs) subject to ventilation and underneath heating by the radiators does not have this resillience.
Using a cartrige to prepare a standard solution of HCl that you rinse into a volumetric flask and fill up with deionised water may be seen valid as alternative for a secondary standard by convention. (ISO standards are settled agreements; similar to ASTM standards, if you see things differently, you may contact them.)  Obviously, alteration of a single step within a whole chain of procedures may invalidate the protocol's fit into ISO; a less severe issue in a teaching lab than for a GMP-accredited lab.
Addendum
&quot;How is the diluted solution of HCl obtained?&quot; brought an other element into play that I did not consider earlier: the use of concentrated hydrochloric acid. If purchased and used freshly, it may be a batch of concentrated (37%) or even &quot;fuming HCl&quot; (about 40%), and both are hygroscopic.  Small errors about the concentration of the stem solution of this corrosive reagent may indeed yield larger systematic errors, down the road of repeated dilutions.  (Obviously not so important if the sole intend is to fill up again the bottle of 1 M or 2 M HCl for the extractive workups ...)