Its possible to construct a transaction that takes up almost 1MB of space and which takes 30 seconds or more to validate on a modern computer (blocks containing such transactions have been mined). In 2MB blocks, a 2MB transaction can be constructed that may take over 10 minutes to validate which opens up dangerous denial-of-service attack vectors. Other lines of code would need to be changed to prevent these problems.
Hmm ... let's write some lines of code to restrict transaction size to let's say 1 MB instead of writing 500 lines of code for Segregated Witness that quadripples the complexicity of bitcoin and brings less capacity to a later point in time?