As I said, insufficient throw weight. In 1999 neither side had more that 25-30 weapons (there are a couple sources that go higher for India, but the overall number never exceeds 50). Simply not enough, especially when you consider that a significant number of the weapons will never reach target (neither side had missiles with enough reach to most of the other. The Indians at the time could reach Karachi, assuming they launched almost at the frontier with Pakistan. The Pakistani's, at the time, were limited to a battlefield launcher with a range of around 100 km. Most deliveries would have been via manned systems, the loss rate on those would be substantial. There is also the matter of mT that would be involved. A single SS-19 could put out the same throw as either of the combatants, a couple Trident D-5s the same (the big boys in this game really are BIG)
Best guess would be 20 maybe 30 actual detonations. Bad, massive death tolls, but the particulates wouldn't be a patch on Mount St. Helen's eruption and would pale compared to Mount Pinatubo with its 2.4 Cubic MILES of particulates (11,000,000,000 TONS) and 17 MILLION tons of sulfur oxide. Nuclear winter (which is more than a bit dodgy as a concept) and Nuclear Autumn (which seems to stand on much more solid ground) assumes, as a minimum requirement 100 city size firestorms. In this scenario, assuming no escalation, that simply isn't happening.