Lots of reasons. It's very underrated, especially here.
1. Excellent world structure and exploration. The maps in BG1 are set up to be relatively adjacent, meaning one flows into the next, e.g. forests on the edge of one will turn into forests on the adjacent edge of the next. This makes it similar to an open world game, and creates a sense of a real region, as opposed to the more common point-of-interest approach in BG2 and many other RPGs. Also, each map is large and filled with wilderness, not just interesting stuff, so that you have to explore around, find interesting stuff.
2. Excellent character progression and itemization. BG1 captures the beauty of character development by starting you off as an absolute weakling and making each level hard to obtain, but providing outstanding rewards for doing so. This is character development at its best. Likewise, it has one of the better itemizations out there. Magic items are rare and usually unique or special in some way, from beautifully written descriptions to back-story to special effects. Even buying a regular non-magic high-end armor will cost a ton of gold and require you to adventure for a while.
3. Well written main story that is well integrated into the game-world. You will find references or tie-ins to the main plot on many optional maps, which makes the whole thing more believable.
4. Beautiful graphics, sounds and AI, leading to a charming atmosphere.
5. A low-key, more realistic world. If you go to an optional side-map, what you find there is more likely to be a farmer with some problems or a waylaid traveler rather than a downed spaceship, or a dragon, or a demi-lich. For some people, that is a good thing.
6. Fun combat system.