The pardon was a favour to the Libertarian Party who have long campaigned for Ulbricht. He promised he would pardon him in return for lending him their votes.
The campaign on behalf of Ulbricht is interesting because it's not really been of interest to the traditional right wing, mostly the libertarian types but also many on the left leaning side of things.
His sentence was considered to be massive overreach and many civil liberties orgs and lawyers became involved. The court case they feel was corrupt due to factors mentioned in sentencing that were not part of the court case so not things he was actually convicted of.
Two of the FBI agents who were involved in the sting operation colluded to steal profits from Silk Road and went to jail for 5 years which makes their evidence immediately problematic.
I think he did deserve prison time but the case and the sentencing became very political and even the prosecutors were shocked at the sentence handed down because it went far beyond what they had asked for.
The pardon will also appeal to the tech bros who supported Trump because it's widely felt that the political overreach was not about Ulbricht's crimes per say but clamping down on the emerging crypto currency market.