Why does "hot chocolate powder" lose taste?

The best before date is often only valid until the pack is opened.
I'd check the pack itself for confirmation, but many goods will say "Best before [2 years]" but then underneath something along the lines of "Once opened keep in [circumstances] & consume within [much shorter time]"

I first looked up shelf life of cocoa powder - essentially it will last several years if kept cool & dry, but will lose flavour long before eventually going off. Long-term it can go rancid & become unsafe, but long before that it will simply lose all its chocolate flavour.
I couldn't find any absolute authoritative citation for this; it was an opinion formed from many sources, quoting similar but not identical information.

Sweet whey powder, on the other hand, has a shelf life of only 6 - 12 months, according to Dairy for Global Nutrition*. Similar anecdotal sources to those for cocoa powder above say this will also lose flavour & eventually smell & taste like milk gone off. [This search is hindered by the number of results for protein whey powder, which rather bury those for sweet whey.]

*Dry dairy products are known for having a neutral, mild dairy flavor. Storage at high temperatures and high relative humidity may increase the potential development of off-flavors. In many cases, significant changes in product flow and solubility will occur before any changes in flavor and aroma become evident. Flavor changes develop more quickly in agglomerated or "instant" ingredients.

^{Emphasis mine}

In conclusion, I imagine that what you tasted was not so much 'less sugar' as less 'chocolate' & potentially slightly past its best sweet whey.