I have answered that question here in detail, the answer also got accepted.
I will provide a section from my answer here and include it in this answer:
He 'used the fire that welled there from the heart of the earth in his
sorceries and his forging.' The most famous result of his forging, and
in fact the only one we know of for sure, was the One Ring.
Mount Doom was much more than just any volcano - Sauron seems to have
extended his own power into it, just as his former master Melkor had
extended his own power into the flesh of Arda as his means of
corrupting the Valar's shaping of the world. In his case, it was
probably due to his use of it as a foundry for the forging of the
Ring, and was able to control its fires. It seems to have lain dormant
when Sauron was away from Mordor, and sprung into life when his power
It's an important place for Sauron as it represents the instrument for his ruling of the world. The more his power grows, the more roaring and fizzing the fires become. We can offer two conclusions. First one, that Sauron's own powers are tied into Mount Doom along with his sorceries. Second one, that he needed Mount Doom for the construction of other items, preservation of his power, form or he was attempting other things with it.
Additionally, we can offer a conclusion that every spawn or forgery of Mount Doom was destroyed or perished with its final eruption...
The world was bent, so that thereafter, only Elven-Ships could sail
into the Utter West. Sauron's body was destroyed, but his spirit was
not diminished, and he fled back to Mordor bearing the Ring, where he
slowly rebuilt a new body and his strength during the time known as
the Dark Years. From this point on, he lost the ability to assume a
fair shape, and ruled now through terror and force.
...even Sauron's ability to assume any kind of shape or form again.
But while Sauron had much of his former strength, he was still much
weakened without the One Ring and remained hidden in the shadows,
directing his armies from afar.
Furthermore, we can conclude that he needed it running in order to forge something else, when he grew more stronger, and tie his strength and power into it once more.
The Road approached the east side of the base at a causeway and then
wound up like a snake; at that point the Road seemed damaged by the
lava and re-repaired several times.
It is also placed deep inside Mordor and thus needn't much protection. He was seeking The Ring with the Eye and flying Nazgul over it as he was rebuilding and wanted to maintain it for himself, only his purposes, whatever they may be. He didn't even consider that someone planned to destroy the One Ring.