Reasons people choose them:

- Total reward return is very competitive
- The 120,000-mile Travel Package is still among the best hotel rewards there is
- Their pattern of paid stays doesn't line up with Starwood...for example, Marriott has North American suburban/rural areas covered far better than Starwood.
- Corporate contracts with Marriott. More of my employers and clients have had their "primary" contracts with Marriott than anyone else.
- Quality of the mainline Marriott/Renaissance brands is more consistent than Hilton/Doubletree.
- Easy to rack up piles of HH and SPG without hotel stays. Therefore, if you do your business travel at Marriott and use one of those other chains' credit cards for your non-Marriott expenses, you can build elite status and large point balances in two programs. Diversification is more important with the hotels than the airlines. (With airlines, a single major alliance can cover most of the world.)

Those are just a few...

I share the sentiment about Marriott doing quirky things to block awards from being used, but even so...I've had more good successes over the years than failures. The key (for me) is finding good international non-resort properties with 7-day lounges where I actually want to spend 6-7 days to use the hotel portion of a Travel Package.