Naruto has been on a steady decline in sales since the war arc started. The final volume sales were a little over a million; by no means a flop, but not what they were probably aiming for.
When the first volume of Boruto didn't break 200k, and continued to decline, that's a bad sign for the franchise moving forward. These numbers may be considered okay for a new monthly series, but for a franchise that was once a major cash-cow, I would've found these numbers unacceptable.
There could be many factors, but if I had to choose:
1) They took too long to get the project off the ground, or didn't wait long enough
It could be either of those things. People WERE getting burned out by Naruto, and Boruto didn't titillate their interest.
On the other hand, SJ DID tease the new era project right before the end of Naruto. Let's face it, everyone was talking about how the series would end and who the children would be (both pro and anti). For better or worse, they moved forward with the Last, the Boruto movie came out afterwards....and for a whole year we heard nothing. That probably killed interest, being promised something but having nothing to show for it.
2) Didn't give off a good first impression
Obviously most were NOT a fan of the artstyle. Good or bad, it was a little bit too different to pass as one of Kishimoto's works. Sarada's outfit made no sense, and they only made it worse as well.
3) REHASH
Probably the biggest factor, and very stupid on their part. Fans got the Boruto movie animated, they didn't want to see it redone in manga format (especially with such a different artstyle). They didn't make a manga version of the Last, if nothing else because they probably realized they would only be showing off what they saw before in a less-appealing format. Add to the fact it's monthly and what should have felt like two hours instead felt like eternity.
4) We have a glimpse of the ending
Yeah, I'm not sure what they were thinking with this one. It felt like they were trying to do what the first episode of Shippuden did, but in the later that manga chapter had been out for a while. Here, NO ONE knows what the hell is going on, and the pacing of the story is not helping.
5) The anime is more interesting
I probably shouldn't put this on the list since I really haven't cared to watch the anime, but I do know that the anime is having their own arcs and so far has not adapted any of the new material from the manga. I wouldn't say these arcs are "good" (apparently the last arc was especially bad) but at least they're animated and have some action going on. It's something at least.
6) Kishimoto is not involved
I think this is probably the main factor. Kishimoto said since the Boruto movie he wasn't going to do Naruto anymore, and I think many hardcore fans took this as a sign to get off. When a creator hands his work over to someone else, the response is anywhere from mixed to negative.