Those areas aren't the same as the urban core of boston-- the areas that remained the nicest after the decline of manufacturing around Boston are generally the places with easy highway access-- W. Roxbury, Newton, Brookline, Arlington, etc. all come to mind. The nicest and most expensive places in MA, are all places with easy relatively easy transportation to skilled jobs.
Haverhill has largely turned around as a result of the proximity to 495 and 93. Lawrence is better than it was, but these areas were completely gutted of industry because they were solely dependent on low-value add activities and lacked the institutions to move up the value chain (as boston has with educational facilities, financial services). Now, the prospects for these areas are the best they have been in years as a result of sprawl and highways-- but the core issue of travel time to the highway/job remains a reason why they aren't attracting more demand from suburbanites that want to live in a more walkable area.
Also, many of the places on the Merrimack were declining in economic power as far back as the 1910s-- as a result of being on a river without major rail or road facilities and far from population centers as well as union agitation. I believe that at one point there was a proposal to dredge the merrimack river all the way to Lowell to make the area more hospitable to business.
Kahta -- this dynamic goes back many many years -- its always about where to live that is pleasant and where to work which often depends on access to long-distance transportation / communications
Boston has always been the Hub for working, transpoting and living in New England -- but until very recently people wanted to move away from where they worked -- hence commuting patterns, traffic and demographics which we have today -- note there have always been a few outliers for various reasons (e.g. WWII based industry) -- but the fundamentals are essentially given by the following:
1) in the 1600's and 1700s working was connected with ships and trade thus Hancock worked over by Faneuil Hall but he lived on Beacon Hill -- other rich forlk did the same in Salem -- think PEM
2) 100 years later -- for people in the upper crust such as Jack Gardner working was shuffling money on State St. -- but you lived on Beacon or Commonwealth in the Back Bay, or you took the commuter rail to home on the North Shore, South Shore or West to Wellesly
3) meanwhile the not so rich involved in white collar work in Boston's FID and managers at the Hub's factories moved to Sommerville, Medford, Arlington and took the troley home
4) Blue collar folks working in the port, or shoes or other small scale manufacturing moved to Southy and Dorchester and took the trolley
5) with the advent of cars and highways the diaspora expanded futher out to R-128 and beyond -- where for the most part there were never any commuters -- just farmers and hence no CR
6) meanwhile starting in 1820's people realized rivers were power and so industry grew in places such as Waltham, Lowell, Lawence, Taunton, Fall River -- providing both power and transport to Boston and then to the rest of the world
7) Late in the 19th Century steam took over for water for power and rail took over from harbors for transport -- the rest of the map of industrial eastern Massachusetts was essentially set by 1900
8) Starting in the early 20th century electricty took over from steam for power and in the post WWII auomobiles and trucks took over from a lot of rail for transport of people and goods
a) many of the mills moved out to places where the raw materials were located (e.g. Carolina) -- thus the downsizing of Lowell, Lawrence, Haverhill, Brockton, Fall River, New Bedford, Lynn
b) later in the 1960's the rest of Blue Collar industrial companies began to leave Boston, Cambridge, Sommerville -- either go far away, or in many cases to move to where there was good highway access and lots of accessible land in the western suburbs to build more effecient industrial facilities
c) some of the big industry such as GE in Lynn transformed to do new things -- but typically they shiftd up the curve to higher skill and higher education employees and typically they got smaller in numbers of employees
9) Starting with DEC -- the minicomputer companies built in the west and northwestern suburbs -- mostly abandoning the biggest cities, and even when they located in places such as Waltham or Lowelll it was in places centric to hightways and away from downtowns and rails -- e.g. Winter St. in Waltham or Wang towers on Rt-3 in Lowell
10) finally New factors which have entered the mix in the past 20 years:
a) information technology and new digital communications companies (some spawed from the Minicomputer companies and some such as Sun / Oracle as imports) are starting to move back inside the Rt-128 loop
b) Bio / Pharma is bound to Cambridge and Boston with some "not quite R and D + production locating out on Rt-128 and up I-93
c) white collar / Knowledge Economy people are starting to want to live in the Hub and susidiary cores such as Waltham Moody St. -- in many cases the only way to get to where they work is by car
d) some of the KE companies are starting to locate in Cambridge and Boston -- offering the potential for transit commuting
e) Huge growth of almost all of the colleges and universities as both the initiation and beneficiries of the KE
Unfortunately - a lot of planning by the various entities -- doesn't seem to realize that plans drawn-up by the T in 1968 are nearly irrelevant today