Why dont our rugby posts just have one post going into the ground?

new hunslet had them at the greyhound stadium

Because with the current rules that would reduce the chances of a Try.Why follow the Yanks anyway.
Rugby was around before American Football.

why change ?

Cartha Queens Park RFC in Glasgow had single post goals for years so obviously is or was allowed.

thousands of pitches with 2 uprights.

Why would you want to change ?

Lower cost of materials
Lower cost of padding the lower 5 feet or so
Fewer injuries from players colliding with the post
Would take about 30 seconds off mowing time for the groundsman

Just because the USA thought of them first, doesn't mean they're a bad idea.

Have you budgeted for the initial cost of purchasing and installing your new fangled posts, the committee meetings it would take to agree on such a change
Also one post- one advertising space

You have not thought this through

I would have thought a single post structure costs more than the H style as would need to be a lot stronger to remain stable.

NFL goose-neck support is 152 mm diameter
IRB upright support is 140 mm diameter ( *2 )

a set of 13metre high posts costs approx £2300+vat.the posts at the large stadiums (wembley etc) are £5000 +vat.It would cost millions to replace all the rugby posts.

You wouldnt have to change them all in one go, just make it the "norm" from now on.

To cc.."why dhange?"...

Well the post in the ground is basically a nuisance, so reducing that nuisance by half would be a good thing imo.  Some interesting points brought up mind, I hadnt considered the cost/advertising space, just the nuisance factor.

*change

Not sure why you think it would reduce by half rob, it would surely be reduced by 9/10ths or more - the goose-neck design means there is one nuisance which is a few metres behind the try line.

The vast majority of pitches and posts have no advertising whatsoever - the few venues that do get meaningful use of the space on the goal post padding could paint the sponsors name in the in-goal area - there's plenty of other creative possibilities but to deliberately put player obstructions on your pitch for the purpose of financial gain when you're replacing your equipment anyway is irresponsible imv.