Why Summer brings frequent thunderstorm activity, and lightning discharges radiate broadband impulse noise that propagates well on 160, 80 and 40 meters. The result is a high static (QRN) floor on the lower HF bands, especially at night when those bands would otherwise be at their best. Weak signals get buried in the crashes, so summer operating on the low bands is usually noisier and shorter-range than winter operating.
Watch out Saying propagation is poor at any time of day overstates it: the ionosphere still supports the band, it is the received noise, not the refraction, that degrades summer low-band work. Worldwide daylight propagation on the low bands is blocked by D-layer absorption, and 'photon absorption' distortion is not a real effect.
Summer thunderstorms equal QRN: low bands get noisy, not dead. Winter is low-band DX season.