Question Why did the British Monarchy stop marrying Brits for 250 years?

[Hierosolyma]

If you look at the British Monarchs from Henry VII (first Tudor monarch) to Queen Anne (last Stuart monarch), you will find double digit amounts of British ancestry. You even see the amount of British ancestry cut in half every generation during the Stuart Era in England, with the exception being the final two Stuart queens, who had a British mother.
I actually looked through their genealogy on Wikitree, and calculated this (rounded to the nearest percent unless it is less than 1%)

BRITISH (English, Scottish, Welsh) ANCESTRY IN BRITISH MONARCHS

HOUSE OF TUDOR

Henry VII - 57% British
Henry VIII - 50% British
Edward VI - 75% British
Mary I - 25% British
Elizabeth I - 75% British

HOUSE OF STUART (aka STEWART)

James VI & I - 48% British
Charles I - 24% British
Charles II, James VII & II - 12% British
William of Orange - 6% British
Mary II, Queen Anne - 56% British

JACOBITE PRETENDERS

The Old Pretender (James VII & II) - 6% British
Bonnie Prince Charlie (Charles III), Cardinal Duke of York (Henry I & IX) - 3% British

HOUSE OF GUELPH (aka HANOVER or ESTE or OBERTENGHI)

George I - 6% British
George II - 3% British
George III - 0.8% British
George IV, William IV - 0.4% British
Victoria - 0.2% British

HOUSE OF WETTIN (aka SAXE-COBURG-GOTHA or WINDSOR)

Edward VII - 0.1% British (the least ethnically British King since the Normans)
George V - 0.2% British
Edward VIII, George VI - 0.3% British
Elizabeth II - 46% British (first monarch to have an ethnically British parent since Queen Anne who died in 1714)

HOUSE OF OLDENBURG (aka MOUNTBATTEN-WINDSOR) including the two likely successors to the current king

Charles III - 23% British (Probably the last British monarch to have a foreign royal as a parent)
William V - 54% British (first monarch with a majority of British ancestry since Queen Anne who died in 1714)
George VII - 77% British

So for the entirety of the Hanoverian era and most of the Wettin/Windsor/Saxe-Coburg/Gotha era, the monarchs had very little actual British ancestry and maintained a custom of marrying foreign royalty instead of British nobility, even though in the Tudor and Stuart eras, marriages to British nobility and even sometimes commoners (Anne Hyde's father only received a peerage after she was married to James VII & II). James VI & I's father, Henry Stuart, Lord Darnley, for example, was a Scottish nobleman.

I understand why the early Hanoverians married other Germans, seeing as they were born and raised in Hanover, and never expected to become the British monarchs, but why did this custom persist all the way up until WWI, when anti-German sentiment finally put a stop to it?

 

[Archetype0ne]

Politics. In monarchies always, but in the middle ages especially, most government forms used marriages to create political ties to other monarchies/dynasties.
Once a dynasty consolidates power in their realm, the incentives shift to creating alliances with other realms.

 

[qh777]

Religion played a big role. The monarchy had issues with Catholics. It’s part of the reason the house of Hanover, George III, became king. I can’t remember if it was him or his dad that was married into the British royalty.

 

[Vallicanus]

Religion played a big role. The monarchy had issues with Catholics. It’s part of the reason the house of Hanover, George III, became king. I can’t remember if it was him or his dad that was married into the British royalty.

Religion did play a big role and still does today. British monarchy still do not marry Catholics.
Pre-Unification North Germany had plenty of Protestant princelings from places like Hanover and Saxe-Coburg-Gotha.

 

[Joey37]

Interestingly, as I am 47% British (English and Welsh, predominantly English), William, who is the same generation as I am (he's four years younger), would be the first monarch with more blood from the actual country he ruled than me, a diaspora, well, half-breed (47 is quite close to 50 so let's not split hairs), as I just barely exceed his grandmother (or equal her, depending on if you rounded up, as I did).

 

[Hierosolyma]

Interestingly, as I am 47% British (English and Welsh, predominantly English), William, who is the same generation as I am (he's four years younger), would be the first monarch with more blood from the actual country he ruled than me, a diaspora, well, half-breed (47 is quite close to 50 so let's not split hairs), as I just barely exceed his grandmother (or equal her, depending on if you rounded up, as I did).

Crazy that you could grab any random African-American off of a USA street and they would almost certainly have more British blood in them than all the British monarchs from 1714-1952 did.

 

[VMax]

Probably looked at their offspring and said - ouch. They realized they needed new blood - that inbreeding had its consequnces. LMAO!