Adding to the discussion,
S/O who is American from the deep Middle America, the heartland....

Explained to me once,
Marriott as an American company has a dedicated American following,
usually Americans prefer to stay at a Marriott because they can expect American conveniances they all know and love.
aka: expectations of strong airconditioning in all seasons weather, icemachine, American hot buffet breakfast, high level English proficiancy, American service standards, consistency. As such, American tour companies, American businesses prefer to have their clients stay in a Marriott.
Any other non-American brand sounds/smells/tastes/reeks foreign, as such hesitant to step out of the comfort zone and book a different chain.

( same logic applies to American chains of Hilton & Hyatt)

And indeed,
in our courting days, (90s/00s)
all Marriotts we stayed in were very consistent,
besides the always present "American" facilities,
all the Marriott decor was consistent red & gold and a certain book of M was always in the drawer of the nightstand.
And the opnion was it is a safe slice of America in a strange country.

In his experience, this changed with
Marriott acquiring / adding new brands, each aimed for different target market.
The once awe inspiring luxury top JW Marriott was surpassed by St. Regis, Edition etc. reducing JW Marriott to a 2nd tier brand
Plus some Marriotts were in a need for refurbishments.
Also Marriott hotels were slowly getting quite expensive, just for the sake of offering the American comforts.
With so many new different branded hotels by Marriott, it was getting difficult to memorize the benefits
It didn't help that with many creditcards and promos came more elites vying for the same benefits and thus encountered some disappointments.
(Earned Platinum was quite sexy in the past, now its forked out as credit candy)
It didn't help also that in some countries, staying in a Marriott was painting a bullseye and volunteering for target practice.
And with having a family, travel requirements / patterns / the budget changed in which Marriott wasn't the best fit for our needs.

And as such we slowly drifted away from Marriott.
If comparing IHG with Marriott,
we find that that with IHG we have more value in the wide range of lower & middle tiers hotels, as that is their core business.
They are developing a lot of fresh new Holiday Inn Express, Voco, Indigo and Kimptons.
There are still dreary Crowne Plaza & Holiday Inns out there but the newer builts outside the US are quite up to date.
But the biggest limitation of IHG is that it has limited reach in upper luxury segment.
An area which Marriott (by adding the SPG hotels) is much stronger in and which appeals to a different demographic targeting aspirational stays only.