(I just answered this in /r/wales)
Huge, huge question, but here's a quick run-down.
The early political opposition to devolution, led by people like Neil Kinnock, was almost exclusively economic. It was about the potential damage that self-reliance would inflict on the Welsh economy, but it got really nasty in the Commons. He came up with all sorts of crap, like telling Parliament that kids in Anglesey were being stopped from going to the toilet unless they asked in Welsh, but people were mostly fearful about what it would do to their household income. The fears weren't limited to traditional unionist voter bases. Throughout the campaign, between 9-12% of Plaid Cymru voters consistently polled for No. There were genuine fears that the Welsh economy would completely collapse, confounded by the decline of traditional industry.
It was also a case of the Yes campaign being woefully under prepared to affect such an emphatic change in British politics. Welsh nationalism suffered from a chronic lack of leadership in the 1970s. I like Dafydd Wigley's quote about Plaid being like a "child preparing for an exam". The Welsh-language pressure group, Cymdeithas Ar Iaith, didn't even bother to campaign, because they felt that Welsh wouldn't be used enough in the proposed Assembly. The press jumped on all of this and failed to back the Yes campaign in any meaningful way. There was nothing to coalesce around, other than little pockets of nationalist support in mid and north Wales. No prominent figurehead steering the ship. Most people in Wales in the 70s read English newspapers, who either barely covered it, or came out for No in-line with their commercial British identities.
This was all compounded by the lack of clarity on what the Assembly would actually be. There was no legislative remit outlined throughout the campaign. Basically, nobody had a fucking clue as to what it was actually going to do, and what powers it would be granted either from the Welsh Office, Local Authorities, or Westminster.
When all's told though, it just made sense to a hell of a lot of people that Wales didn't have the political pedigree to go its own way, and the economic consequences of doing so were just too severe. In the end, it was a devastating loss.
| Area | % of electorate voting Yes (% of turnout) | % of electorate voting No |
|---|---|---|
| Clwyd | 21 (21.6) | 40.1 (78.4) |
| Gwynedd | 21.8 (34.4) | 41.6 (65.6) |
| Dyfed | 18.1 (28.1) | 46.5 (71.9) |
| Powys | 12.2 (18.5) | 53.8 (81.5) |
| West Glamorgan (go on boys) | 10.8 (18.7) | 46.7 (81.3) |
| South Glamorgan | 11.8 (20.2) | 46.7 (79.8) |
| Gwent | 7.7 (13.1) | 51 (86.9) |
| 6.7 (12.1) | 48.7 (87.9) |
Edit - Spelling and added a table with election returns.
________________________
Martin Johnes, Wales Since 1939 (Manchester: Manchester University Press, 2012), p. 295-298.
To answer why wales voted against devolution in 1979 we need to look at the political and socioeconomic state of Wales and the UK, as well as the devolution issue itself.
The late 1970s in the UK was defined by economic problems - under the labour prime minster Callaghan, public expenditure was slashed and deflationary policies introduced to deal with double digit inflation. Callaghan presided over the Winter of Discontent in 1978/79 which reduced the popularity of the Labour Party, in favour of Thatcher’s Conservatives. As many jobs in South Wales were dependent on coal and the value this created, the closure of the collieries had a greater effect on the South Wales economy than the UK as a whole.
Nationalism was an extremely divisive subject in Wales at the time. The majority of the population were (and still are) centred around the South East of the country, which had greater links to the rest of the UK. Cymraeg was spoken by around a fifth of the country in 1979, mainly in the North West and South West of the country. In the majority English speaking areas, the Welsh Language Society was seen at best as an eccentric fringe movement to be ignored, at worst as a waste of money and attempt to categorise English speakers as second class citizens. The Free Wales army also created a deal of revulsion within the majority of the country. Plaid Cymru campaigning for independence also added to this division between those who saw themselves as British - the majority of the country at the time would have lived through, or served in, the war and saw themselves as British first.
The offer to Wales on the referendum was fairly poor from a divided UK parliament. There was to be little, if any, power sitting within the Senedd, and a turnout threshold of 40% was required to pass. Popular politicians, like future Labour leader Kinnock, campaigned for No on issues like cost. With the economic issues Wales and the UK were facing, the Parliament was not seen as important compared with what the money allotted could be spent on. The Nationalist support also pushed people away from Yes, as they were opposed by a majority within South East Wales. The referendum was lost of a 20.3%/79.7% on a 59% turnout.
These feelings were also present in the 1997 referendum, which passed with 50.3% on a turnout of 50%, however the campaign was supported by the Labour, Lib Dem, and Plaid parties which were the main parties in Wales at the time. The support for the Senedd was along the same lines with a strong majority in Welsh speaking areas and the South East of the country voting against.