Because he's holding his hands out to catch his lightsaber. He pulls the lightsaber towards himself then goes to catch it, Jango grapples his hands just before he catches it. It would be pretty daft of Obi-Wan to dodge the grappling hook and therefore also miss catching his lightsaber. The best plan is to hope the lightsaber reaches him before the grappling hook so he can attack Jango. Unfortunately for Obi-Wan, Jango wins the race and grapples him before his lightsaber can reach his hands but only just before.
If you actually watch the scene slowed down Obi-Wan does end up catching his lightsaber as the grappling hook has wrapped his hands. However, he gets pulled along by Jango just before he gets a good grip and so it goes flying.
All in all he did the most favourable outcome for himself which was to arm himself with his weapon so he wasn't a sitting duck. He just missed out by a couple of seconds.