You're not alone, I felt the exact same way. I recognize that it's an awesome game but I only beat it once when it came out, and every time I try to go back I get bored for some reason... HL1 on the other hand I can revisit any time and still love it. I think my problem for me is that the Combine were kind of boring and bland as an enemy, in fact the enemies roster in general did not intrigue me. Your allies were okay but I didn't think the City 17 and the rebellion angle was all that interesting.
Fighting my way out of Black Mesa in HL1 and watching the situation slowly and discretely escalate was a lot more engaging. Even the HL1 remake, Black Mesa, is infinitely better to me than HL2, so I think it just comes down to how the story is presented (show don't tell) and how it progresses, as well as the mystery, the enemy roster etc. I simply think it's a more satisfying journey overall.
It is indeed an unpopular opinion but I'll stand by it. They are both top-tier games regardless of how I feel about them.
That said, Black Mesa is a fantastic project and if you like HL1 you should definitely check it out. It's pretty amazing how well they fleshed out the world without deviating too far from the original vision.
