Surely there is no reason for them not to open it on 17th? This would be pretty bad since everyone's already (rightly or wrongly) booked to visit
Personally I think Alton Towers have been very sensible to stick to "Spring 2018" as the opening date, as this gives them flexibility to open the ride only once it is really ready & running with a high reliability %rate. As has already been said above, look at The Smiler & DBGT that both had advertised opening dates that were missed, with associated negative publicity. Think back to 2002 when Air opened on the 1st day of the season & ran with lots of reliability issues for it's first season - if they'd delayed Airs' opening by a week or so many of these issues could have been resolved.
Many parks are suitably vague about opening dates to give them flexibility to iron out issues. Islands of Adventure opened the Wizarding World of Harry Potter in "Spring" - this turned out to be June 18th (as the first day of Summer is considered to be 21st June - so they met the Spring deadline to keep the lawyers off their backs, Americans love to sue for anything).
I do think it is highly likely we'll see Wicker Man open very early in the season, if not on March 17th - it first test-ran in mid-December. Alton Towers are just being sensible and covering their backs, as negative publicity is something that they can do without as the resort continues it's recovery.