The game was released in 1999. It was coded for 1999 computers. The source code was released in 2001 and since then a lot of work has gone into the code, but the SCP crew aren't miracle makers.
I'm sure you understand that some parts of the code remain essentially untouched, with some limitations of the original game still in effect.
I'm not in the SCP crew so I don't know if this particular limit is inherited from the retail game or if it is of some later origin, but it really doesn't matter. Coders don't put limits into code just for the hell of it, although convenience might be a reason for some occasions (and it's perfectly valid reason if dynamic limit isn't feasible and you make sure the program won't exceed the limit). If you think it's so easy to up limits, download the source and implement the changes, then show them to the SCP crew and if they're good they might just get included.
Also, dropping code efficiency in favour of new features.

I would facepalm, but my face has been a bit worn out recently. I'm sure you understand why.