There's a lot of Pokemon that can hit it super effectively. Timburr, Mienfoo, Pawniard, Grookey, Magnemite, and more are all fairly commonplace, and as such Lileep takes a big hit from all their super effective moves. It's Knock Off bait (Grookey, Pawniard, a lot of others in LC), and Pawniard can come in easily to set up, as Eviolite Pawniard isn't too fearful of a non-STAB Earth Power (plus it's immune to Toxic and resists Giga Drain). Any priority after a Knock Off pretty much kills it (Pawniard and Grookey say hello again), so there goes your Lileep.
It is actually pretty good for a Water resist, being able to sponge up Mareanie and Staryu Scald. However, so are Dewpider and Frillish, both of which bring more to the table. Frillish gets helpful utility with Scald and Will-O-Wisp, plus more coverage (though it is weak to Knock Off). Dewpider gets the amazing Sticky Web for a dedicated lead set.
In short, Lileep is hit by a lot super effectively, and it's outclassed by other Water immunities.
Hope I helped!