Hydrochloric acid is a limiting reagent.
The key here is the balanced chemical equation that describes this redox reaction
#"Na"_ 2"S"_ 2"O"_ (3(aq)) + color(red)(2)"HCl"_ ((aq)) -> "NaCl"_ ((aq)) + "S"_ ((s)) darr + "SO"_ (2(g)) uarr + "H"_ 2"O"_((l))#
Notice that the reaction consumes
This means that in order for the reaction to consume both reactants completely, you must have twice as many moles of hydrochloric acid than you have of sodium thiosulfate.
Now, you're mixing equal volumes,
This means that you have fewer moles of hydrochloric acid than you'd need to allow for all the moles of sodium thiosulfate to react
The reaction will stop because the hydrochloric acid will be completely consumed before all the moles of sodium thiosulfate get the chance to take part in the reaction.
As you can see in the video below, this reaction is often used to show how concentration affects reaction rate