If you mean waveguide slot array antenna , then this is because of the weight and cost of the waveguide structure! Actually waveguide slot antenna is very heavy to construct.

A patch antenna (also known as a rectangular microstrip antenna) is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane. The assembly is usually contained inside a plastic radome, which protects the antenna structure from damage. Patch antennas are simple to fabricate and easy to modify and customize

A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation distribution pattern. Often the radio waves are provided by a waveguide, and the antenna consists of slots in the waveguide. Slot antennas are often used at UHF and microwave frequencies instead of line antennas when greater control of the radiation pattern is required. Slot antennas are widely used in radar antennas, for the sector antennas used for cell phone base stations, and are often found in standard desktop microwave sources used for research purposes.

A slot antenna's main advantages are its size, design simplicity, robustness, and convenient adaptation to mass production using PC board technology.

Also, the Article given below

Ultra Wideband Slotted Microstrip Patch Antenna for Downlink and Uplink Satellite

Application in C band

Tajeswita Gupta and P. K. Singhal

Department of electronics and communication MITS Gwalior

Gwalior, M.P. 474005, India

LINK:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCsQFjAA&url=http%3A%2F%2Fwww.issr-journals.org%2Fxplore%2Fijias%2FIJIAS-13-111-01.pdf&ei=FeouUpzYIYTXrQeh_oAo&usg=AFQjCNFBLtqOyY7ySkW6bwk-64cLFHC8zg&bvm=bv.51773540,d.bmk

A novel temple shaped antenna with various slots has been presented in the paper. Proposed antenna achieves the two bands in the IEEE C band for downlink and uplink applications in satellite communication. The achieved bands are 3.077-4.9

GHz & 6.98-7.67 GHz. The bandwidth for lower band is 1.8 GHz which provides it an ultra-wideband application for the achieved band. The proposed antenna has shown Low Profile characteristic and better impedance bandwidth and Return Loss performance.

A Microstrip patch antenna is a narrowband, wide- beam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating substrate. Because such Antennas have a very low profile, are mechanically rugged and can be conformable, they are often mounted on the exterior of aircrafts and spacecrafts, or are incorporated into mobile radio communications devices.

Various slots have been introduced in the patch to achieve wider bandwidth.

P.S.

but on slot antennas are used...it depends on the platform

Smaller size, less weight, but also two or more patch antennas plus some extra circuits = dynamic phased array (electronically steerable antenna). I do not think you can put morer than one slot array antenna (size again).