I would guess it's because (1) they want to pioneer safe-standing and (2) they are hoping for promotion to the Championship, where for the time being it is all-seater. They are in the League 1 play-off final.
Strangely enough, if they were already in the Champ they would not be allowed to even install the rail seats, but I guess that if they already have them when they come up, they can keep them and use them in the seat positions only.
See this quote from the article below:
As Championship teams are not permitted to install safe-standing, Shrewsbury had previously believed the rail seats would require installation before League One's play-off final at Wembley on Sunday, May 27.
But the English Football League have revealed that, should Town be successful in reaching the second tier, promotion would be officially confirmed at the EFL AGM in Portugal on June 7.
Town raised the £65,000 funding via crowdfunding experts Tifosy last year after the Supporters' Parliament-led bid was first revealed last June.
Following detailed talks and planning with the Sports Ground Safety Authority (SGSA) and Shropshire Council's Safety Advisory Group (SAG), work is now ready to begin.