Because Gradinarov substituted V. Nikolov in the group match vs the Netherlands, and he played surprisingly well.
The substitution of Aleksiev with Salparov has happened a couple of times in this championship. Plamen might want Aleksiev who's no longer that young to take a rest, and at the same time to improve the reception of the team. Aleksiev doesn't make pipe attack that often.
That's logical but we are talking about the decisive moments of the match. Nikolov was not a reliable spiker at that time so taking out Aleksiev was very risky and actually counter-productive.