I assume that you're looking at Consumer Reports for the information that you have? Consumer reports is NOT a reliable source of information when it comes to car seat testing. Their testing methods aren't released to public or even the car seat manufacturers, and there are a number of concerns with their recommendations. They aren't car seat experts, to put it mildly.
The Evenflo port-a-bout car seats aren't being made anymore, so that one isn't really an issue IMO. It's important to note though, that government testing didn't show this problem, and that it only occurred for consumer reports when the base was installed with LATCH.
All car seats sold in Canada meet the same minimum standards, and there is no way to know if one is safer than another or which one performs best. The car seat approval process in CAnada does differ from the US. In the US, car seat manufacturers do their own testing and basically self certify that their seats meet standards and then the NHTSA does spot checks and investigates problems.
In Canada, a car seat manufacturer develops a seat, then send the bare seat to Transport Canada. The seat is then tested by a government agency to ensure that it meets all CMVSS standards. Provided it meets CMVSS standards, then the government issues a letter to the manufacturer giving them permission to sell seats in Canada and affix the CMVSS sticker to their seats. Any modification to the shell of the seat itself requires the process to be repeated. Fabric covers and harnesses themselves are not included in the government process, but the government has standards governing covers and harness straps that the manufacturer must conform to.
Compliance testing is also done periodically by the government to ensure standards are continually being met. Recalls are issued when problems are found that pose a safety risk in a collision. Other times notices are issued about manual instructions or an issue that doesn't pose a safety risk, but consumers need to be aware of.
As to why the government doesn't develop a 5 star rating system or something similar, I can't give a precise answer. What I can tell you is that the 5 star rating system you see with car manufacturers is assigned by the IIHS - Insurance Institute of Highway Safety, and they do their own testing. When it comes to vehicles, the government mandates minimums, and it's up to manufacturers how far beyond minimums they go. With cars, it's easier to judge if one should be safer than another. Independent agencies do more testing, and some safety features would seem to obviously offer a greater deal of protection than a car without those safety features...
I'm not sure how completely that answers your question, I can't really tell you "why," I can only tell you how the the certification process is done. With car seats you will find some features that would seem like they would provide more protection in a collision - EPS/EPP foam is perhaps the most obvious example, as well as true side impact protection (TSIP,) like you see in the Britax Parkway booster. (We don't have any convertible seats that offer true side impact protection, so that's why I'm singling out the Parkway even though it's a booster. The Britax Boulevard does have TSIP, but is not available in Canada.)
In the end, the safest car seat is one that fits your vehicle, fits your child and will be used correctly each and every trip. It doesn't always matter how much safer one car seat is than another - if it's not being used properly, it's going to be safer than if you had one that just met the minimum standards but was being used correctly. Not every car seat is compatible with every vehicle, and some seats are outgrown by a child long before they're ready for the next step. A child will be safer in a 5pt harness at the age of 6 - even one without EPS/EPP foam and no side impact protection, than they would be in a booster that had both.
