I'd say prize money and sponsorship plays it's part. Someone like Marat has made a lot of money from tennis - he could retire now and be well off for the rest of his life. Don't forget, the players also get appearance money - they can turn up, lose first round and still make a lot of money - especially if they are amongst the 'popular' players.
Also, maybe the 'traditional element' from tennis maybe lacking. Monte Carlo has been down-graded from next year - and yet in the 1980's/90's, it was a prestigious tournament to win. Maybe today's players think of tournaments in order of money on offer? not on the great players in the past who won them.
All good points. Money is a major culprit. When there is no sense of urgency, as in...I have to win so I can put food on the table or a roof over my head,
it's far easier to become complacent.
When you can make a fabulous salary by most measures, buy lots of toys, and enjoy the perks of a great standard of living without killing yourself, the motivation doesn't necessarily manifest itself. Some players are content to just finance a lifestyle, not put forth 100% of their effort.