Dear Santokh ji
I dont know whether you know this or not
Even Today there are Gurdwara where neat is served . I have myself taken Goat meat in Langar In delhi in gurdwara Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/interfaith-dialogues/18095-why-no-meat-served-in-gurdwara.html
While in Uk I have met many people from Afghan who informed that that Meat is no taboo to thier culture and gurdwara
I n India Due to heavy influenceof Vaishnite and also need of the time as nearly 20 % devotees to any gurdwara even Hrimandar Sahib are non sikhs even today so there needs to be taken care off
When the langra was started at our guruji time the main devotees were either Hindu or Muslim and both had contrasting Diets although there were non Veg Hindus but Muslim wont eat Non Halal meat so the rreason it was eliminated from Langar probably
This fact is even more brought to clarity by the fact that Sevapanthi preacher who used to preach to muslim were veg only due to fact that NON Halal meat is not acceptable to Muslims
Why are 3HO and Gora Sikhs vegetarians ?
for 3 HO Sikhs Bhajan Singh Yogi ji was Water shed Line and they cant cross it at anty cost so they are mostly vegetrain due to him and also the Kundalini Yoga they practice
Why are Nirmala Sikhs vegetarians ?
As I know from various sources like Vijaydeep singh who is regular Vistor to Nirmala Akharas They are not gainst Meat eating and allow the house holder to eat meat
Why are Namdharis vegetarians ?
This group has Living Guru and they Follow instructions according ly
Why are Radha Soamis vegetarians ?
This not a Sikh gropu by any Dimension so need to comment
Why are AKJ's vegetarians ?
This group started by Bhai Randhir Singh ji is league of its own and mantian very strict maryada which is differtnt from main Stream maryada Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=18095
they just add to diversity of sikh culture . and mind you Meat is one of the minor Issue compred to many contrasting Idea this group has
the Group you didnt mentioned are
Taksal Sikhs - they are also vegetarian
Nihang singh group - they are Mixed Group
Their are two kinds of them some are vegetarian and other non vegetarian but those vegetarian dont make non veg a issue like AKJ
And yes you are Right Randip Singh is THE EXPERT on the Issue!