Why are Terminal Alkynes less reactive than internal alkynes?
The mechanism of hydration is an electrophilic addition and requires polarization of the carbon-carbon triple bond. This task is more difficult compared to case of alkene due to it having more %s character
In case of internal alkynes the carbocation intermediate formed ( since this mechanism is electrophilic addition the intermediate is a carbocation) is stabilized by the hyperconjugative effect and the inductive effect of the alpha carbons.
This support is taken away in case of terminal alkynes as their is no alpha-carbon. Now terminal alkynes can still be hydrated but only in presence of $\ce{Hg^2+}$ ion in acidic medium because of $\ce{Hg^2+}$'s high polarizing power(because it has a pseudo inert configuration). This polarization succeeds in catalyzing the reaction.
Ref.: Morrison and Boyd, sec 12.10 (hydration of alkynes)