Genuine question, why do Cardiff and Swansea (and Wrexham and Newport I think?) play in the English leagues? As far as I know there aren't aren't Scottish or Northern Irish teams in the English setup, so why do Welsh clubs?
When the football league was formed there wasn't a Welsh League for them to join, so playing in the English one was the only option. Those teams you mention plus Merthyr and Conwyn Bay I think. Can only assume Northern Ireland and Scotland had their own leagues.
Gretna ( Scotland) played in the English football system till 2002. So could have achieved English football league status through the pyramid.
The real question is why don't Scottish and Northern Irish/Ulster teams play in the same league with the English/Welsh teams? Would make it a lot more interesting in my opinion. It's too late now because of the whole system I guess but in an ideal world I'd say just have one British league.
Travel was the issue in the old days when leagues were formed . Lots of players working day after and same day on lots of occasions .
The Football League was formed in 1888 and Cardiff and Swansea were invited to join about 100 year ago. The Football League was rebranded as the English Football League as late as 2016. I don't think there was ever a question of Cardiff and Swansea and Newport not being included as they'd been in that League for almost 100 years. The Welsh Premier League was formed in 1992 - now know as Cymru Premier League. Apart from one club all clubs are Welsh based. The exception is England based TNS (Total Network Solutions) which is based in Oswestry Shropshire. TNS the most successful team but current Champion is Connah's Quay. Connah's Quay has a ground capacity of only 1400.
That's a really dumb reason. Have you seen the MLS or even just bigger European countries? Aren't CD Tenerife in the Spanish league? Cagliari in the Italian league etc.. Plus in the British hockey league Belfast, Glasgow and Cardiff all have teams.. Doesn't seem to be a problem. The UK is tiny there's no way travel could ever be a reason.
I think that UEFA and FIFA don't see the UK as four countries, they see it as one. The four Great British/UK/whatever nations get four votes on important matters like rule-changes, and it's seen as a historical anomaly. They would prefer that a Great Britain/UK/whatever team represents the island in international football, and in the early 90s, they warned the Welsh FA that they would take away their right to play international football if they didn't form a domestic football league. Cardiff and the like were asked if they wanted to join. They'd get regular Cup Winners' Cup (now Europa League) qualification as a carrot if they did, but they refused. So the League of Wales is formed of basically village teams, and the teams from bigger Welsh towns and cities continued in the English league. I think this is correct, anyway.
When English league was formed it was pretty localised, mainly in NWest and only went as far down as Birmingham, by the time first club ‘down south’ joined in Woolwich Arsenal the Scottish one was established.
A few other points that may have played a part in the original governance of the league: Northern Ireland didn't exist when the league was created - it was still a united Ireland. Scotland is a separate country and has it's own law and legal system. Wales was conquered by England and subsquently adopted English laws.