..because they're losing their best finisher in Fionn Carr and have no quality depth.
Assuming they lose Fionn Carr, what other options do they have on the wing? Liam Bibo is ****, Troy Nathan is a squad player, Shane Monahan hasn't done anything and both Darragh Fanning and Brian Tuohy are little more than AIL players. Hudson is a prolific try scoring wing with a big future ahead of him but he needs gametime which Leinster can't provide. A back three of Duffy, O'Halloran and Hudson (or Dave Kearney) is infinitely better than Connacht having to play any of the others I've listed here.
Eoin Griffin is a decent prospect, of that there's no doubt. Their other 13 is an injury prone 30 year old Niva Ta'auso. At present they're so stuck for 13s that Ian Keatley has to plug into that position. Macken is undoubtedly gifted but is stuck behind Brian O'Driscoll, Luke Fitzgerald, Eoin O'Malley and Fergus McFadden in the queue for Leinster's outside centre spot. Why should he bide his time playing AIL and A level rugby when he can go to Connacht, fight for a regular starting spot and improve his skill set? It works for Connacht because they need added competition for places, competition which doesn't exist at present. Beyond their starting XV, Connacht have no quality. This is especially true in the centre and second row where even the starters aren't much use.