This is Inflation, Pure and Simple
GW2’s original price tag was $59.99, which is standard for new games. HoT is $49.99, which is the standard for full expansions. $40 was the standard for MMO expansions with subscriptions. The last WoW expansion was $49.99, the others were $40.
The GW1 campaigns were $59.99 and treated as standalone games. EotN was $34.99 and GW1’s only actual expansion.
HoT is simply priced to that of the standard. Whether it actually offers the amount of content you would expect out of a full priced expansion is another question.