This is a very tough problem to solve as these kinds of decisions can be made by a multitude of different factors. I am going to attempt to answer it to the best of my ability.
Many times these bloated and expensive enterprise suites are purchased with minimal involvement by technically oriented personel on the team. Sales demonstrations and demo apps are tools to sell the idea to the functional managers and stakeholders of the project or company.
This problem occurs when the functional managers are given too much power or voice, or technical personel are not given enough. This comes full circle to my previous comment that this may be purposeful but that is another topic of discussion.
Technical involvement and evaluation COULD minimize this problem.
In regards to your idea about a good evaluation site of commercial software it would be EXTREMELY HARD to set such a site up in an UNBIASED or pure way.
Technical evaluations that PRAISE one product over another are worth a LOT of money and an influx of money by concerned parties would put pressure and strain on such a site to stay biased. As if this weren't enough, there are a number of different online services that hire people to put up fake blogs and fake "grassroots" product evaluations that makes identifying TRUELY UNBIASED ratings hard to do. Pay-per-post companies hire people to flood artificial popular opinion on a number of different sites as well.
And yet still, ANOTHER problem you would have to contend with are potential legal liabilities and challenges from companies that own products that received BAD reviews, claiming slanderous falsehoods and loss of business. Many companies (cough Computer Associates) have armies of lawyers that do just that.
These are probably a number of issues why such a site would be hard to run.