Two reasons.
First, I currently don’t get consistent performance from my Enlites which you are experiencing. I’m glad when I do make it to day six. I’m on day six for the current sensor and the ISIG was around 15 for a 100 SG. When I started the ISIG was more like 25 to 29.
The sensitivity (?) of the probe seems to degrade somewhat steadily over time for me. I’m not quite sure if this is a problem with the probe’s chemistry or, I think more likely, because my sensor probes get “kinked up” the longer I wear them.
Note:
I decided to break this out and post it as a separate topic. My intention was to spin it off as a “linked reply”. But for reasons which escape me, the linkage didn’t get set up correctly. So, I added my own kludgy link to my new thread: Slow, progressive Enlite ISIG degradation as the days pass 
Second, I was mistakenly sent 6 boxes of 5 Enlites each instead of the single box I was ostensibly prescribed. While I do not have to pay out of pocket for this, it still infuriates me because it is stupid beyond any words I have and a general PITA.
On the one hand, I’m don’t have to worry about resupplying during the next 6 days * 6 boxes * 5 per box ~= 180 days after I received them.
On the other hand, Enlites never have an expiration date longer than 6 months from their date of manufacturer. My 26 remaining unused Enlites all “expire” 90 days from now on August 13 and at 6 days each I’ll have only used ~15 of them by then. The last ~11 Enlites will have technically expired before I can use them. If any of them completely wonk out on me, I won’t have the option of asking Medtronic to replace it.
Bottom line is that I am not “incentivized” to try to use them for longer than the 6 days Medtronic specifies. 
Oh, well … at least I’ve got CGM. 
No point obsessing any more than I already do about the details. 