"Why don't current missions already use small masers..."
So we have already been using conventional traveling wave tube amplifiers for microwave communications in space. Ka band is around 30 GHz. That's microwave amplification (the "MA" in MASER) and so don't need stimulated emission.
The electronics and amplifiers (TWTAs or semiconductor) are small enough and powerful enough already. A 5 meter diameter MASER would have the same narrow beam and gain of a 5 meter diameter parabolic reflector, and you still need to have the parabolic reflector to receive signals.
"...or IR lasers?"
I think the premise that infrared lasers haven't and/or won't be used for optical communications with deep space will not be easily supported.
According to Design of the ESA Optical Ground Station for Participation in LLCD (Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2012, 3-1, Ajaccio, Corsica, France, October 9-12 (2012) Copyright (c) ICSOS 2012.)
There is so much existing, mature, reliable, well-characterized technology for the optical communications bands around 900, 1300 and 1550 nm (lasers, modulators, amplifiers, fibers, receivers, etc.) that these are the wavelength bands we're likely to "see" in deep space and cis-lunar free-space optical communications.
The use of the DPSS 1064 nm rather than a semiconductor laser and standard optical communications band was a surprise, I don't think we'll see that again for high speed communications, though it's a great pulsed laser for LIDAR and a great beacon wavelength for optical target acquisition before the high speed communications begins.
We'd be fighting an uphill battle pushing into visible light and getting the same quality and reliability that already exists ubiquitously in near IR.