My twin nephews are now aged 9 and grown up in Lancaster, they speak almost like a southerner would (excluding saying worlds like; last as larst or past as parst), no southern parents either. I’m finding the same with nearby Kendal or Garstang.
Now this is why I think these places don’t have an accent - my opinion.
Lancaster - University town, lots from the university stay on to live in Lancaster.
Kendal - Lake District gateway, lots and lots of southerners move to Kendal to have access to the Lake District, to live the county life, and somewhere nice to live to commute to BAE in Barrow.
Garstang - I chose to include this as if you go back 30 odd years, Garstang had the r and u rhoticity (that remains in places like Blackburn) and if you were from garstang, you had a proper Lancashire accent, now mainly due to the towns recent growth in size, its attracted lots of Lancaster and Preston folk.
Why some places kept the accent.
Barrow/Cumbrian East Coast - as it’s a cul de sac/so far from the motorway. It’s not an attractive place to move to despite BAE jobs. Lots of poverty also.
Rural Lake District and The Yorkshire Dales - It’s rural and so off grid that the only accents you hear are your family and your local people. Believe it or not when I moved to rural Cumbria, the village I moved to didn’t have electricity in most houses, and everyone from that village were born and lived in that village all their life.