I would not say "without reason" but rather "because of status with hotel chain". In Bangkok I get suite upgrades at the following hotels based on status, provided that suites are available at time of check-in of course:

St.Regis
W
JW Marriott
Athenee
Sheraton Grande Sukhumvit
Madi Paidi (Autograph Collection)
Waldorf Astoria
Park Hyatt
Kimpton Maa-Lai
Sofitel SO
Banyan Tree

Not sure if you consider these brands 5* or not, but I would also note that I also get suite upgrades at these hotels as well:
Grand Hyatt
Hyatt Regency
Marriott Surawongse
Le Meridien (in the city)
Le Meridien (at the airport)

So that's a very long list of generous hotels and I might be forgetting a few, consequently Sofitel Sukhumvit not providing suite upgrades to Diamonds (if that is still the case), sticks out like a sore thumb and is not competitive in this city. Another notoriously stingy hotel in Bangkok is InterContinental so when I have a particularly stay where I care about the upgrade, I avoid IC and Sofitel Sukhumvit and there are plenty of other valid choices to choose from.