Laziness, lack of care, genuine travel issues, personal reasons - hard to tell for strangers on a forum
But there is a lot of emphasis on everyone attending milestone birthdays in general and not everyone thinks that’s a great idea
To go from very few visitors throughout the year(s) to a houseful on 1 specific day of 1 specific year has the potential to be overwhelming
Why not suggest they visit at another time? If it is travel issues that would allow them to pick the best day for them to travel
And means mother gets multiple visits with time to talk to each person rather than being faced with a wall of people & only a few minutes with each
(have seen people standing around talking to everyone except the birthday person on past occasions)
The added benefit is that no-one is feeling ‘forced’ to be there, ‘stressed’ by travel, or ‘pissed off’ at having had to strong-arm others to come.
Slightly also - lots of assumptions that children are avoiding the mum - could quite easily be they are avoiding siblings