if a character insists on being mysterious after a while they are simply undeveloped and boring. The other part is that Boreanaz has never been a strong actor and didn't even really become any good until he was on his own series.
Plus it's a thankless task to be 'the love interest'. (Also see Riley - at least Angel could brood and be angsty, Riley got nothin'.)

By comparison while one may or may not like Spike he is hardly short of personality or opinions. And yes, he was both on the show longer and working with the Scoobies more extensively. For example between S1 and S2 Buffy is absent but one assumes Angel has had no contact with them either -- which is rather different from S5 to S6 in particular.
Spike will show up and ask for help from them, just because. He lives with Xander, looks after Dawn - he's embedded in a way Angel never is.