MSE is 'definition' or 'signature' based, which means it detects malware with a KNOWN set of characteristics and/or code.
The cretins who push this stuff onto the 'Net don't just sit back a see what happens with one virus: they constantly modify their payloads to improve efficiency, or most often to avoid detection.
They may use pre-existing 'packages' and swap out modules that give it a new dimensions to propagate; or perhaps cloak it's 'mother-ship' with encryption.
It may be something as simple as changing the name of the payload.
(When one method of infecting machines works with resounding success, why abandon it? They just tweak it.)
They also monitor hacker forums to see what other hackers around the globe have uncovered in the way of heretofore unknown security flaws, which opens entirely fresh methods to infect Operating Systems. These are known as 'zero-day' exploits.
So, when you rely on 'signature' based anti-anything, you essentially are one step behind the bad guys, because the 'white-hats' haven't back engineered the newly discovered payloads (that incessantly get pushed out); nor have there been countermeasures developed. You must wait for those to arrive in your 'updates'.
(Another type of anti-virus, called "heuristic", looks at the behavior of a 'program' or instruction set, but those tend to give false positives due to the replication of legit routines being incorporated into all programs...good or evil.)
If you commit to using Windows on the Internet, you must become a security expert and concentrate your efforts towards prevention rather than remediation, or suffer at the hands of those more intelligent than yourself. It's that simple.