That is something that can be explained, if an album is truly great.
How so?
By laying out your criteria for what makes a great album and then showing that the album in question met or exceeded several of those criteria. For example, I might say "Some of the criteria I feel make a 'great album' are: solid album sales, more than one high charting single, accessibility across musical genres, songs with varying structures, songs with strong vocals, songs that are catchy, and some larger social impact that transcends music." (not saying those are the only criteria, but by way of example only)
Let's apply that formula to Michael Jackson's
Thriller:
-Solid album sales? Check. It smashed records and became the highest selling album of all time (at the time, anyway).
-Several high-charting singles? Check. 7 top 10's. Nobody had ever done that before.
-Cross-genre appeal? Check. It lit up pop, R&B, and rock charts.
-Varying song structures? For pop music, check. But your mileage and buy-in may vary with this factor.
-Strong vocals? Check. Michael had range and power, and also showed a lot of diversity and use of different styles.
-Catchy songs? Check. Little explanation required.
-Social impact? "Thriller enabled Jackson to break down racial barriers in pop music via his appearances on MTV and meeting with President of the United States Ronald Reagan at the White House." Not to mention the fact that it, along with some other things, led to a lot of other things that transcended music, such as the USA for Africa charity (remember the We Are The World song/video?). Checkeroo.
Anyhow, you get the idea.
In contrast, let's do Lulu, by Lou Reed and Metallica.
-Solid album sales? Nope.
-Several high-charting singles? Nope.
-Cross-genre appeal? Er...okay, let's say "yes" just for argument's sake even though I think it might be a stretch to say there was
any fona fide "appeal."
-Varying song structures? Again, I'll give this one.
-Strong vocals? Mmm...okay.
-Catchy songs? Not really, by most people's standards.
-Social impact? Zero.
So, great album by my criteria? Not even close. It was a stretch to say that really any of my criteria were met.
Anyhow, this is requiring much more explanation that it should. A lot of people like the album in question, no doubt. Few if any actively dislike it. That's all well and good. I haven't heard why it's great. But if you like it, more power to you.