There are various reasons for why, many have been well documented here.
Please don't forget that BA will be saving millions through this programme as cadets are paid less than DEPs and will also be paying for TRs (at cost price, not commercial like Ryanair impose). Employing low-hours pilots is risky, so BA are ensuring they have a very high quality of cadet they are happy with.
By engaging with prospective pilots before they commence a course the tax bill is reduced through ROTE.
It is "win-win" for wanabees and BA.